Merge pull request #2289 from ctiller/footprints-on-the-sands-of-time

Introduce multiple clocks to GPR

32 files changed, 182 insertions, 115 deletions

diff --git a/src/core/channel/census_filter.c b/src/core/channel/census_filter.c
index 43ef5fb3ff..d996c3475e 100644
--- a/src/core/channel/census_filter.c
+++ b/src/core/channel/census_filter.c
@@ -151,7 +151,7 @@ static void client_init_call_elem(grpc_call_element* elem,
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   init_rpc_stats(&d->stats);
-  d->start_ts = gpr_now();
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   d->op_id = census_tracing_start_op();
   if (initial_op) client_mutate_op(elem, initial_op);
 }
@@ -169,7 +169,7 @@ static void server_init_call_elem(grpc_call_element* elem,
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   init_rpc_stats(&d->stats);
-  d->start_ts = gpr_now();
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   d->op_id = census_tracing_start_op();
   if (initial_op) server_mutate_op(elem, initial_op);
 }
@@ -177,8 +177,8 @@ static void server_init_call_elem(grpc_call_element* elem,
 static void server_destroy_call_elem(grpc_call_element* elem) {
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
-  d->stats.elapsed_time_ms =
-      gpr_timespec_to_micros(gpr_time_sub(gpr_now(), d->start_ts));
+  d->stats.elapsed_time_ms = gpr_timespec_to_micros(
+      gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), d->start_ts));
   census_record_rpc_server_stats(d->op_id, &d->stats);
   census_tracing_end_op(d->op_id);
 }
diff --git a/src/core/client_config/subchannel.c b/src/core/client_config/subchannel.c
index 6cf9062ab0..8cdad1015f 100644
--- a/src/core/client_config/subchannel.c
+++ b/src/core/client_config/subchannel.c
@@ -300,7 +300,7 @@ static void continue_connect(grpc_subchannel *c) {
 }
 
 static void start_connect(grpc_subchannel *c) {
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   c->next_attempt = now;
   c->backoff_delta = gpr_time_from_seconds(1);
 
@@ -585,7 +585,7 @@ static void subchannel_connected(void *arg, int iomgr_success) {
     c->have_alarm = 1;
     c->next_attempt = gpr_time_add(c->next_attempt, c->backoff_delta);
     c->backoff_delta = gpr_time_add(c->backoff_delta, c->backoff_delta);
-    grpc_alarm_init(&c->alarm, c->next_attempt, on_alarm, c, gpr_now());
+    grpc_alarm_init(&c->alarm, c->next_attempt, on_alarm, c, gpr_now(GPR_CLOCK_REALTIME));
     gpr_mu_unlock(&c->mu);
   }
 }
diff --git a/src/core/iomgr/iomgr.c b/src/core/iomgr/iomgr.c
index 4a2c45a023..cca92d3b32 100644
--- a/src/core/iomgr/iomgr.c
+++ b/src/core/iomgr/iomgr.c
@@ -59,7 +59,7 @@ static void background_callback_executor(void *ignored) {
   while (!g_shutdown) {
     gpr_timespec deadline = gpr_inf_future;
     gpr_timespec short_deadline =
-        gpr_time_add(gpr_now(), gpr_time_from_millis(100));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100));
     if (g_cbs_head) {
       grpc_iomgr_closure *closure = g_cbs_head;
       g_cbs_head = closure->next;
@@ -67,7 +67,8 @@ static void background_callback_executor(void *ignored) {
       gpr_mu_unlock(&g_mu);
       closure->cb(closure->cb_arg, closure->success);
       gpr_mu_lock(&g_mu);
-    } else if (grpc_alarm_check(&g_mu, gpr_now(), &deadline)) {
+    } else if (grpc_alarm_check(&g_mu, gpr_now(GPR_CLOCK_REALTIME),
+                                &deadline)) {
     } else {
       gpr_mu_unlock(&g_mu);
       gpr_sleep_until(gpr_time_min(short_deadline, deadline));
@@ -89,7 +90,7 @@ void grpc_iomgr_init(void) {
   gpr_thd_id id;
   gpr_mu_init(&g_mu);
   gpr_cv_init(&g_rcv);
-  grpc_alarm_list_init(gpr_now());
+  grpc_alarm_list_init(gpr_now(GPR_CLOCK_REALTIME));
   g_root_object.next = g_root_object.prev = &g_root_object;
   g_root_object.name = "root";
   grpc_iomgr_platform_init();
@@ -110,26 +111,27 @@ void grpc_iomgr_shutdown(void) {
   grpc_iomgr_object *obj;
   grpc_iomgr_closure *closure;
   gpr_timespec shutdown_deadline =
-      gpr_time_add(gpr_now(), gpr_time_from_seconds(10));
-  gpr_timespec last_warning_time = gpr_now();
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(10));
+  gpr_timespec last_warning_time = gpr_now(GPR_CLOCK_REALTIME);
 
   gpr_mu_lock(&g_mu);
   g_shutdown = 1;
   while (g_cbs_head != NULL || g_root_object.next != &g_root_object) {
-    if (gpr_time_cmp(gpr_time_sub(gpr_now(), last_warning_time),
-                     gpr_time_from_seconds(1)) >= 0) {
-    if (g_cbs_head != NULL && g_root_object.next != &g_root_object) {
-      gpr_log(GPR_DEBUG,
-              "Waiting for %d iomgr objects to be destroyed and executing "
-              "final callbacks",
-              count_objects());
-    } else if (g_cbs_head != NULL) {
-      gpr_log(GPR_DEBUG, "Executing final iomgr callbacks");
-    } else {
-      gpr_log(GPR_DEBUG, "Waiting for %d iomgr objects to be destroyed",
-              count_objects());
-    }
-    last_warning_time = gpr_now();
+    if (gpr_time_cmp(
+            gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), last_warning_time),
+            gpr_time_from_seconds(1)) >= 0) {
+      if (g_cbs_head != NULL && g_root_object.next != &g_root_object) {
+        gpr_log(GPR_DEBUG,
+                "Waiting for %d iomgr objects to be destroyed and executing "
+                "final callbacks",
+                count_objects());
+      } else if (g_cbs_head != NULL) {
+        gpr_log(GPR_DEBUG, "Executing final iomgr callbacks");
+      } else {
+        gpr_log(GPR_DEBUG, "Waiting for %d iomgr objects to be destroyed",
+                count_objects());
+      }
+      last_warning_time = gpr_now(GPR_CLOCK_REALTIME);
     }
     if (g_cbs_head) {
       do {
@@ -150,9 +152,9 @@ void grpc_iomgr_shutdown(void) {
     if (g_root_object.next != &g_root_object) {
       int timeout = 0;
       gpr_timespec short_deadline =
-          gpr_time_add(gpr_now(), gpr_time_from_millis(100));
+          gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100));
       while (gpr_cv_wait(&g_rcv, &g_mu, short_deadline) && g_cbs_head == NULL) {
-        if (gpr_time_cmp(gpr_now(), shutdown_deadline) > 0) {
+        if (gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), shutdown_deadline) > 0) {
           timeout = 1;
           break;
         }
diff --git a/src/core/iomgr/pollset_posix.c b/src/core/iomgr/pollset_posix.c
index 12496440de..85101764d2 100644
--- a/src/core/iomgr/pollset_posix.c
+++ b/src/core/iomgr/pollset_posix.c
@@ -122,7 +122,7 @@ static void finish_shutdown(grpc_pollset *pollset) {
 
 int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   /* pollset->mu already held */
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   if (gpr_time_cmp(now, deadline) > 0) {
     return 0;
   }
@@ -187,15 +187,16 @@ void grpc_pollset_destroy(grpc_pollset *pollset) {
   gpr_mu_destroy(&pollset->mu);
 }
 
-int grpc_poll_deadline_to_millis_timeout(gpr_timespec deadline, gpr_timespec now) {
+int grpc_poll_deadline_to_millis_timeout(gpr_timespec deadline,
+                                         gpr_timespec now) {
   gpr_timespec timeout;
   static const int max_spin_polling_us = 10;
   if (gpr_time_cmp(deadline, gpr_inf_future) == 0) {
     return -1;
   }
   if (gpr_time_cmp(
-        deadline, 
-        gpr_time_add(now, gpr_time_from_micros(max_spin_polling_us))) <= 0) {
+          deadline,
+          gpr_time_add(now, gpr_time_from_micros(max_spin_polling_us))) <= 0) {
     return 0;
   }
   timeout = gpr_time_sub(deadline, now);
diff --git a/src/core/iomgr/pollset_windows.c b/src/core/iomgr/pollset_windows.c
index 8d6bc79c96..24226cc980 100644
--- a/src/core/iomgr/pollset_windows.c
+++ b/src/core/iomgr/pollset_windows.c
@@ -70,7 +70,7 @@ void grpc_pollset_destroy(grpc_pollset *pollset) {
 
 int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   gpr_timespec now;
-  now = gpr_now();
+  now = gpr_now(GPR_CLOCK_REALTIME);
   if (gpr_time_cmp(now, deadline) > 0) {
     return 0 /* GPR_FALSE */;
   }
@@ -86,8 +86,6 @@ int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   return 1 /* GPR_TRUE */;
 }
 
-void grpc_pollset_kick(grpc_pollset *p) {
-  gpr_cv_signal(&p->cv);
-}
+void grpc_pollset_kick(grpc_pollset *p) { gpr_cv_signal(&p->cv); }
 
 #endif /* GPR_WINSOCK_SOCKET */
diff --git a/src/core/iomgr/tcp_client_posix.c b/src/core/iomgr/tcp_client_posix.c
index 2c6ff4e9c4..dc0489e64f 100644
--- a/src/core/iomgr/tcp_client_posix.c
+++ b/src/core/iomgr/tcp_client_posix.c
@@ -253,7 +253,8 @@ void grpc_tcp_client_connect(void (*cb)(void *arg, grpc_endpoint *ep),
   ac->write_closure.cb_arg = ac;
 
   gpr_mu_lock(&ac->mu);
-  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now());
+  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac,
+                  gpr_now(GPR_CLOCK_REALTIME));
   grpc_fd_notify_on_write(ac->fd, &ac->write_closure);
   gpr_mu_unlock(&ac->mu);
 
diff --git a/src/core/iomgr/tcp_client_windows.c b/src/core/iomgr/tcp_client_windows.c
index b1a169b519..16741452b9 100644
--- a/src/core/iomgr/tcp_client_windows.c
+++ b/src/core/iomgr/tcp_client_windows.c
@@ -215,7 +215,8 @@ void grpc_tcp_client_connect(void (*cb)(void *arg, grpc_endpoint *tcp),
   ac->refs = 2;
   ac->aborted = 0;
 
-  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now());
+  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac,
+                  gpr_now(GPR_CLOCK_REALTIME));
   socket->write_info.outstanding = 1;
   grpc_socket_notify_on_write(socket, on_connect, ac);
   return;
diff --git a/src/core/profiling/timers_preciseclock.h b/src/core/profiling/timers_preciseclock.h
index 163d52b797..5c251b47e6 100644
--- a/src/core/profiling/timers_preciseclock.h
+++ b/src/core/profiling/timers_preciseclock.h
@@ -82,7 +82,7 @@ struct grpc_precise_clock {
   gpr_timespec clock;
 };
 static void grpc_precise_clock_now(grpc_precise_clock* clk) {
-  clk->clock = gpr_now();
+  clk->clock = gpr_now(GPR_CLOCK_REALTIME);
 }
 #define GRPC_PRECISE_CLOCK_FORMAT "%ld.%09d"
 #define GRPC_PRECISE_CLOCK_PRINTF_ARGS(clk) \
diff --git a/src/core/security/credentials.c b/src/core/security/credentials.c
index 52fd5a25b3..230f0dfb85 100644
--- a/src/core/security/credentials.c
+++ b/src/core/security/credentials.c
@@ -357,7 +357,8 @@ static void jwt_get_request_metadata(grpc_credentials *creds,
     if (c->cached.service_url != NULL &&
         strcmp(c->cached.service_url, service_url) == 0 &&
         c->cached.jwt_md != NULL &&
-        (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration, gpr_now()),
+        (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration,
+                                   gpr_now(GPR_CLOCK_REALTIME)),
                       refresh_threshold) > 0)) {
       jwt_md = grpc_credentials_md_store_ref(c->cached.jwt_md);
     }
@@ -374,7 +375,8 @@ static void jwt_get_request_metadata(grpc_credentials *creds,
       char *md_value;
       gpr_asprintf(&md_value, "Bearer %s", jwt);
       gpr_free(jwt);
-      c->cached.jwt_expiration = gpr_time_add(gpr_now(), c->jwt_lifetime);
+      c->cached.jwt_expiration =
+          gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), c->jwt_lifetime);
       c->cached.service_url = gpr_strdup(service_url);
       c->cached.jwt_md = grpc_credentials_md_store_create(1);
       grpc_credentials_md_store_add_cstrings(
@@ -545,7 +547,8 @@ static void on_oauth2_token_fetcher_http_response(
   status = grpc_oauth2_token_fetcher_credentials_parse_server_response(
       response, &c->access_token_md, &token_lifetime);
   if (status == GRPC_CREDENTIALS_OK) {
-    c->token_expiration = gpr_time_add(gpr_now(), token_lifetime);
+    c->token_expiration =
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), token_lifetime);
     r->cb(r->user_data, c->access_token_md->entries,
           c->access_token_md->num_entries, status);
   } else {
@@ -567,8 +570,9 @@ static void oauth2_token_fetcher_get_request_metadata(
   {
     gpr_mu_lock(&c->mu);
     if (c->access_token_md != NULL &&
-        (gpr_time_cmp(gpr_time_sub(c->token_expiration, gpr_now()),
-                      refresh_threshold) > 0)) {
+        (gpr_time_cmp(
+             gpr_time_sub(c->token_expiration, gpr_now(GPR_CLOCK_REALTIME)),
+             refresh_threshold) > 0)) {
       cached_access_token_md =
           grpc_credentials_md_store_ref(c->access_token_md);
     }
@@ -582,7 +586,7 @@ static void oauth2_token_fetcher_get_request_metadata(
     c->fetch_func(
         grpc_credentials_metadata_request_create(creds, cb, user_data),
         &c->httpcli_context, pollset, on_oauth2_token_fetcher_http_response,
-        gpr_time_add(gpr_now(), refresh_threshold));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), refresh_threshold));
   }
 }
 
diff --git a/src/core/security/google_default_credentials.c b/src/core/security/google_default_credentials.c
index cd92f9dc30..f622deff42 100644
--- a/src/core/security/google_default_credentials.c
+++ b/src/core/security/google_default_credentials.c
@@ -103,9 +103,10 @@ static int is_stack_running_on_compute_engine(void) {
 
   grpc_httpcli_context_init(&context);
 
-  grpc_httpcli_get(&context, &detector.pollset, &request,
-                   gpr_time_add(gpr_now(), max_detection_delay),
-                   on_compute_engine_detection_http_response, &detector);
+  grpc_httpcli_get(
+      &context, &detector.pollset, &request,
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), max_detection_delay),
+      on_compute_engine_detection_http_response, &detector);
 
   /* Block until we get the response. This is not ideal but this should only be
      called once for the lifetime of the process by the default credentials. */
diff --git a/src/core/security/json_token.c b/src/core/security/json_token.c
index 2e2b980129..9b1ea255ae 100644
--- a/src/core/security/json_token.c
+++ b/src/core/security/json_token.c
@@ -207,7 +207,7 @@ static char *encoded_jwt_claim(const grpc_auth_json_key *json_key,
   grpc_json *child = NULL;
   char *json_str = NULL;
   char *result = NULL;
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   gpr_timespec expiration = gpr_time_add(now, token_lifetime);
   char now_str[GPR_LTOA_MIN_BUFSIZE];
   char expiration_str[GPR_LTOA_MIN_BUFSIZE];
@@ -218,8 +218,8 @@ static char *encoded_jwt_claim(const grpc_auth_json_key *json_key,
   gpr_ltoa(now.tv_sec, now_str);
   gpr_ltoa(expiration.tv_sec, expiration_str);
 
-  child = create_child(NULL, json, "iss", json_key->client_email,
-                       GRPC_JSON_STRING);
+  child =
+      create_child(NULL, json, "iss", json_key->client_email, GRPC_JSON_STRING);
   if (scope != NULL) {
     child = create_child(child, json, "scope", scope, GRPC_JSON_STRING);
   } else {
@@ -403,4 +403,3 @@ void grpc_auth_refresh_token_destruct(grpc_auth_refresh_token *refresh_token) {
     refresh_token->refresh_token = NULL;
   }
 }
-
diff --git a/src/core/security/jwt_verifier.c b/src/core/security/jwt_verifier.c
index 01007a1a84..9140eb2ef7 100644
--- a/src/core/security/jwt_verifier.c
+++ b/src/core/security/jwt_verifier.c
@@ -189,7 +189,6 @@ struct grpc_jwt_claims {
   gpr_slice buffer;
 };
 
-
 void grpc_jwt_claims_destroy(grpc_jwt_claims *claims) {
   grpc_json_destroy(claims->json);
   gpr_slice_unref(claims->buffer);
@@ -286,12 +285,14 @@ grpc_jwt_verifier_status grpc_jwt_claims_check(const grpc_jwt_claims *claims,
 
   GPR_ASSERT(claims != NULL);
 
-  skewed_now = gpr_time_add(gpr_now(), grpc_jwt_verifier_clock_skew);
+  skewed_now =
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), grpc_jwt_verifier_clock_skew);
   if (gpr_time_cmp(skewed_now, claims->nbf) < 0) {
     gpr_log(GPR_ERROR, "JWT is not valid yet.");
     return GRPC_JWT_VERIFIER_TIME_CONSTRAINT_FAILURE;
   }
-  skewed_now = gpr_time_sub(gpr_now(), grpc_jwt_verifier_clock_skew);
+  skewed_now =
+      gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), grpc_jwt_verifier_clock_skew);
   if (gpr_time_cmp(skewed_now, claims->exp) > 0) {
     gpr_log(GPR_ERROR, "JWT is expired.");
     return GRPC_JWT_VERIFIER_TIME_CONSTRAINT_FAILURE;
@@ -327,10 +328,10 @@ typedef struct {
 
 /* Takes ownership of the header, claims and signature. */
 static verifier_cb_ctx *verifier_cb_ctx_create(
-    grpc_jwt_verifier *verifier, grpc_pollset *pollset,
-    jose_header * header, grpc_jwt_claims *claims, const char *audience,
-    gpr_slice signature, const char *signed_jwt, size_t signed_jwt_len,
-    void *user_data, grpc_jwt_verification_done_cb cb) {
+    grpc_jwt_verifier *verifier, grpc_pollset *pollset, jose_header *header,
+    grpc_jwt_claims *claims, const char *audience, gpr_slice signature,
+    const char *signed_jwt, size_t signed_jwt_len, void *user_data,
+    grpc_jwt_verification_done_cb cb) {
   verifier_cb_ctx *ctx = gpr_malloc(sizeof(verifier_cb_ctx));
   memset(ctx, 0, sizeof(verifier_cb_ctx));
   ctx->verifier = verifier;
@@ -604,7 +605,7 @@ end:
 
 static void on_openid_config_retrieved(void *user_data,
                                        const grpc_httpcli_response *response) {
-  const grpc_json* cur;
+  const grpc_json *cur;
   grpc_json *json = json_from_http(response);
   verifier_cb_ctx *ctx = (verifier_cb_ctx *)user_data;
   grpc_httpcli_request req;
@@ -632,9 +633,10 @@ static void on_openid_config_retrieved(void *user_data,
   } else {
     *(req.host + (req.path - jwks_uri)) = '\0';
   }
-  grpc_httpcli_get(&ctx->verifier->http_ctx, ctx->pollset, &req,
-                   gpr_time_add(gpr_now(), grpc_jwt_verifier_max_delay),
-                   on_keys_retrieved, ctx);
+  grpc_httpcli_get(
+      &ctx->verifier->http_ctx, ctx->pollset, &req,
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), grpc_jwt_verifier_max_delay),
+      on_keys_retrieved, ctx);
   grpc_json_destroy(json);
   gpr_free(req.host);
   return;
@@ -645,8 +647,8 @@ error:
   verifier_cb_ctx_destroy(ctx);
 }
 
-static email_key_mapping *verifier_get_mapping(
-    grpc_jwt_verifier *v, const char *email_domain) {
+static email_key_mapping *verifier_get_mapping(grpc_jwt_verifier *v,
+                                               const char *email_domain) {
   size_t i;
   if (v->mappings == NULL) return NULL;
   for (i = 0; i < v->num_mappings; i++) {
@@ -733,9 +735,10 @@ static void retrieve_key_and_verify(verifier_cb_ctx *ctx) {
     http_cb = on_openid_config_retrieved;
   }
 
-  grpc_httpcli_get(&ctx->verifier->http_ctx, ctx->pollset, &req,
-                   gpr_time_add(gpr_now(), grpc_jwt_verifier_max_delay),
-                   http_cb, ctx);
+  grpc_httpcli_get(
+      &ctx->verifier->http_ctx, ctx->pollset, &req,
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), grpc_jwt_verifier_max_delay),
+      http_cb, ctx);
   gpr_free(req.host);
   gpr_free(req.path);
   return;
@@ -764,7 +767,7 @@ void grpc_jwt_verifier_verify(grpc_jwt_verifier *verifier,
   dot = strchr(cur, '.');
   if (dot == NULL) goto error;
   json = parse_json_part_from_jwt(cur, dot - cur, &header_buffer);
-  if (json == NULL)  goto error;
+  if (json == NULL) goto error;
   header = jose_header_from_json(json, header_buffer);
   if (header == NULL) goto error;
 
@@ -772,7 +775,7 @@ void grpc_jwt_verifier_verify(grpc_jwt_verifier *verifier,
   dot = strchr(cur, '.');
   if (dot == NULL) goto error;
   json = parse_json_part_from_jwt(cur, dot - cur, &claims_buffer);
-  if (json == NULL)  goto error;
+  if (json == NULL) goto error;
   claims = grpc_jwt_claims_from_json(json, claims_buffer);
   if (claims == NULL) goto error;
 
@@ -827,4 +830,3 @@ void grpc_jwt_verifier_destroy(grpc_jwt_verifier *v) {
   }
   gpr_free(v);
 }
-
diff --git a/src/core/statistics/census_rpc_stats.c b/src/core/statistics/census_rpc_stats.c
index 0491c91947..3e571b1143 100644
--- a/src/core/statistics/census_rpc_stats.c
+++ b/src/core/statistics/census_rpc_stats.c
@@ -157,7 +157,7 @@ static void record_stats(census_ht* store, census_op_id op_id,
         key.ptr = gpr_strdup(key.ptr);
         census_ht_insert(store, key, (void*)window_stats);
       }
-      census_window_stats_add(window_stats, gpr_now(), stats);
+      census_window_stats_add(window_stats, gpr_now(GPR_CLOCK_REALTIME), stats);
     } else {
       census_internal_unlock_trace_store();
     }
@@ -185,7 +185,7 @@ static void get_stats(census_ht* store, census_aggregated_rpc_stats* data) {
   if (store != NULL) {
     size_t n;
     unsigned i, j;
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     census_ht_kv* kv = census_ht_get_all_elements(store, &n);
     if (kv != NULL) {
       data->num_entries = n;
diff --git a/src/core/statistics/census_tracing.c b/src/core/statistics/census_tracing.c
index 05e72b99c0..3036ba5407 100644
--- a/src/core/statistics/census_tracing.c
+++ b/src/core/statistics/census_tracing.c
@@ -94,7 +94,7 @@ census_op_id census_tracing_start_op(void) {
     g_id++;
     memcpy(&ret->id, &g_id, sizeof(census_op_id));
     ret->rpc_stats.cnt = 1;
-    ret->ts = gpr_now();
+    ret->ts = gpr_now(GPR_CLOCK_REALTIME);
     census_ht_insert(g_trace_store, op_id_as_key(&ret->id), (void*)ret);
     gpr_log(GPR_DEBUG, "Start tracing for id %lu", g_id);
     gpr_mu_unlock(&g_mu);
@@ -122,7 +122,7 @@ void census_tracing_print(census_op_id op_id, const char* anno_txt) {
   trace = census_ht_find(g_trace_store, op_id_as_key(&op_id));
   if (trace != NULL) {
     census_trace_annotation* anno = gpr_malloc(sizeof(census_trace_annotation));
-    anno->ts = gpr_now();
+    anno->ts = gpr_now(GPR_CLOCK_REALTIME);
     {
       char* d = anno->txt;
       const char* s = anno_txt;
@@ -143,8 +143,8 @@ void census_tracing_end_op(census_op_id op_id) {
   gpr_mu_lock(&g_mu);
   trace = census_ht_find(g_trace_store, op_id_as_key(&op_id));
   if (trace != NULL) {
-    trace->rpc_stats.elapsed_time_ms =
-        gpr_timespec_to_micros(gpr_time_sub(gpr_now(), trace->ts));
+    trace->rpc_stats.elapsed_time_ms = gpr_timespec_to_micros(
+        gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), trace->ts));
     gpr_log(GPR_DEBUG, "End tracing for id %lu, method %s, latency %f us",
             op_id_2_uint64(&op_id), trace->method,
             trace->rpc_stats.elapsed_time_ms);
@@ -194,8 +194,8 @@ const char* census_get_trace_method_name(const census_trace_obj* trace) {
 
 static census_trace_annotation* dup_annotation_chain(
     census_trace_annotation* from) {
-  census_trace_annotation *ret = NULL;
-  census_trace_annotation **to = &ret;
+  census_trace_annotation* ret = NULL;
+  census_trace_annotation** to = &ret;
   for (; from != NULL; from = from->next) {
     *to = gpr_malloc(sizeof(census_trace_annotation));
     memcpy(*to, from, sizeof(census_trace_annotation));
@@ -223,9 +223,9 @@ census_trace_obj** census_get_active_ops(int* num_active_ops) {
     size_t n = 0;
     census_ht_kv* all_kvs = census_ht_get_all_elements(g_trace_store, &n);
     *num_active_ops = (int)n;
-    if (n != 0 ) {
+    if (n != 0) {
       size_t i = 0;
-      ret = gpr_malloc(sizeof(census_trace_obj *) * n);
+      ret = gpr_malloc(sizeof(census_trace_obj*) * n);
       for (i = 0; i < n; i++) {
         ret[i] = trace_obj_dup((census_trace_obj*)all_kvs[i].v);
       }
diff --git a/src/core/statistics/window_stats.h b/src/core/statistics/window_stats.h
index d733d8d247..0020f6b44c 100644
--- a/src/core/statistics/window_stats.h
+++ b/src/core/statistics/window_stats.h
@@ -90,11 +90,11 @@
     // Record a new event, taking 15.3ms, transferring 1784 bytes.
     stat.latency = 0.153;
     stat.bytes = 1784;
-    census_window_stats_add(stats, gpr_now(), &stat);
+    census_window_stats_add(stats, gpr_now(GPR_CLOCK_REALTIME), &stat);
     // Get sums and print them out
     result[kMinInterval].statistic = &sums[kMinInterval];
     result[kHourInterval].statistic = &sums[kHourInterval];
-    census_window_stats_get_sums(stats, gpr_now(), result);
+    census_window_stats_get_sums(stats, gpr_now(GPR_CLOCK_REALTIME), result);
     printf("%d events/min, average time %gs, average bytes %g\n",
            result[kMinInterval].count,
            (my_stat*)result[kMinInterval].statistic->latency /
@@ -170,4 +170,4 @@ void census_window_stats_get_sums(const struct census_window_stats* wstats,
    assertion failure). This function is thread-compatible. */
 void census_window_stats_destroy(struct census_window_stats* wstats);
 
-#endif  /* GRPC_INTERNAL_CORE_STATISTICS_WINDOW_STATS_H */
+#endif /* GRPC_INTERNAL_CORE_STATISTICS_WINDOW_STATS_H */
diff --git a/src/core/support/cancellable.c b/src/core/support/cancellable.c
index 5a4d488dd3..3cbb318ab6 100644
--- a/src/core/support/cancellable.c
+++ b/src/core/support/cancellable.c
@@ -121,8 +121,8 @@ void gpr_cancellable_cancel(gpr_cancellable *c) {
         } else {
           gpr_event ev;
           gpr_event_init(&ev);
-          gpr_event_wait(&ev,
-                         gpr_time_add(gpr_now(), gpr_time_from_micros(1000)));
+          gpr_event_wait(&ev, gpr_time_add(gpr_now(GPR_CLOCK_REALTIME),
+                                           gpr_time_from_micros(1000)));
         }
       }
     } while (failures != 0);
diff --git a/src/core/support/log_linux.c b/src/core/support/log_linux.c
index 7937466b79..5ac36e7b95 100644
--- a/src/core/support/log_linux.c
+++ b/src/core/support/log_linux.c
@@ -76,7 +76,7 @@ void gpr_default_log(gpr_log_func_args *args) {
   char *prefix;
   const char *display_file;
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   final_slash = strrchr(args->file, '/');
diff --git a/src/core/support/log_posix.c b/src/core/support/log_posix.c
index afca792c40..940ee20f15 100644
--- a/src/core/support/log_posix.c
+++ b/src/core/support/log_posix.c
@@ -75,7 +75,7 @@ void gpr_default_log(gpr_log_func_args *args) {
   char *final_slash;
   const char *display_file;
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   final_slash = strrchr(args->file, '/');
diff --git a/src/core/support/log_win32.c b/src/core/support/log_win32.c
index d249be7d2e..629781da8a 100644
--- a/src/core/support/log_win32.c
+++ b/src/core/support/log_win32.c
@@ -82,7 +82,7 @@ void gpr_log(const char *file, int line, gpr_log_severity severity,
 /* Simple starter implementation */
 void gpr_default_log(gpr_log_func_args *args) {
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   if (localtime_s(&tm, &now.tv_sec)) {
diff --git a/src/core/support/sync_win32.c b/src/core/support/sync_win32.c
index 72f39f8d46..29b77fc4c2 100644
--- a/src/core/support/sync_win32.c
+++ b/src/core/support/sync_win32.c
@@ -86,7 +86,7 @@ int gpr_cv_wait(gpr_cv *cv, gpr_mu *mu, gpr_timespec abs_deadline) {
   if (gpr_time_cmp(abs_deadline, gpr_inf_future) == 0) {
     SleepConditionVariableCS(cv, &mu->cs, INFINITE);
   } else {
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     gpr_int64 now_ms = now.tv_sec * 1000 + now.tv_nsec / 1000000;
     gpr_int64 deadline_ms =
         abs_deadline.tv_sec * 1000 + abs_deadline.tv_nsec / 1000000;
diff --git a/src/core/support/time_posix.c b/src/core/support/time_posix.c
index afb58ef231..f9b7958783 100644
--- a/src/core/support/time_posix.c
+++ b/src/core/support/time_posix.c
@@ -55,22 +55,52 @@ static gpr_timespec gpr_from_timespec(struct timespec ts) {
   return rv;
 }
 
-gpr_timespec gpr_now(void) {
+/** maps gpr_clock_type --> clockid_t for clock_gettime */
+static clockid_t clockid_for_gpr_clock[] = {CLOCK_MONOTONIC, CLOCK_REALTIME};
+
+void gpr_time_init(void) {}
+
+gpr_timespec gpr_now(gpr_clock_type clock) {
   struct timespec now;
-  clock_gettime(CLOCK_REALTIME, &now);
+  clock_gettime(clockid_for_gpr_clock[clock], &now);
   return gpr_from_timespec(now);
 }
 #else
 /* For some reason Apple's OSes haven't implemented clock_gettime. */
 
 #include <sys/time.h>
+#include <mach/mach.h>
+#include <mach/mach_time.h>
+
+static double g_time_scale;
+static uint64_t g_time_start;
+
+void gpr_time_init(void) {
+  mach_timebase_info_data_t tb = {0, 1};
+  mach_timebase_info(&tb);
+  g_time_scale = tb.numer;
+  g_time_scale /= tb.denom;
+  g_time_start = mach_absolute_time();
+}
 
-gpr_timespec gpr_now(void) {
+gpr_timespec gpr_now(gpr_clock_type clock) {
   gpr_timespec now;
   struct timeval now_tv;
-  gettimeofday(&now_tv, NULL);
-  now.tv_sec = now_tv.tv_sec;
-  now.tv_nsec = now_tv.tv_usec * 1000;
+  double now_dbl;
+
+  switch (clock) {
+    case GPR_CLOCK_REALTIME:
+      gettimeofday(&now_tv, NULL);
+      now.tv_sec = now_tv.tv_sec;
+      now.tv_nsec = now_tv.tv_usec * 1000;
+      break;
+    case GPR_CLOCK_MONOTONIC:
+      now_dbl = (mach_absolute_time() - g_time_start) * g_time_scale;
+      now.tv_sec = now_dbl * 1e-9;
+      now.tv_nsec = now_dbl - now.tv_sec * 1e9;
+      break;
+  }
+
   return now;
 }
 #endif
@@ -83,7 +113,7 @@ void gpr_sleep_until(gpr_timespec until) {
   for (;;) {
     /* We could simplify by using clock_nanosleep instead, but it might be
      * slightly less portable. */
-    now = gpr_now();
+    now = gpr_now(GPR_CLOCK_REALTIME);
     if (gpr_time_cmp(until, now) <= 0) {
       return;
     }
diff --git a/src/core/support/time_win32.c b/src/core/support/time_win32.c
index 9db267c01b..fa77c74eeb 100644
--- a/src/core/support/time_win32.c
+++ b/src/core/support/time_win32.c
@@ -40,12 +40,34 @@
 #include <grpc/support/time.h>
 #include <sys/timeb.h>
 
-gpr_timespec gpr_now(void) {
+static LARGE_INTEGER g_start_time;
+static double g_time_scale;
+
+void gpr_time_init(void) {
+  LARGE_INTEGER frequency;
+  QueryPerformanceFrequency(&frequency);
+  QueryPerformanceCounter(&g_start_time);
+  g_time_scale = 1.0 / frequency.QuadPart;
+}
+
+gpr_timespec gpr_now(gpr_clock_type clock) {
   gpr_timespec now_tv;
   struct _timeb now_tb;
-  _ftime_s(&now_tb);
-  now_tv.tv_sec = now_tb.time;
-  now_tv.tv_nsec = now_tb.millitm * 1000000;
+  LARGE_INTEGER timestamp;
+  double now_dbl;
+  switch (clock) {
+    case GPR_CLOCK_REALTIME:
+      _ftime_s(&now_tb);
+      now_tv.tv_sec = now_tb.time;
+      now_tv.tv_nsec = now_tb.millitm * 1000000;
+      break;
+    case GPR_CLOCK_MONOTONIC:
+      QueryPerformanceCounter(&timestamp);
+      now_dbl = (timestamp.QuadPart - g_start_time.QuadPart) * g_time_scale;
+      now_tv.tv_sec = (time_t)now_dbl;
+      now_tv.tv_nsec = (int)((now_dbl - (double)now_tv.tv_sec) * 1e9);
+      break;
+  }
   return now_tv;
 }
 
@@ -57,13 +79,14 @@ void gpr_sleep_until(gpr_timespec until) {
   for (;;) {
     /* We could simplify by using clock_nanosleep instead, but it might be
      * slightly less portable. */
-    now = gpr_now();
+    now = gpr_now(GPR_CLOCK_REALTIME);
     if (gpr_time_cmp(until, now) <= 0) {
       return;
     }
 
     delta = gpr_time_sub(until, now);
-    sleep_millis = (DWORD)delta.tv_sec * GPR_MS_PER_SEC + delta.tv_nsec / GPR_NS_PER_MS;
+    sleep_millis =
+        (DWORD)delta.tv_sec * GPR_MS_PER_SEC + delta.tv_nsec / GPR_NS_PER_MS;
     Sleep(sleep_millis);
   }
 }
diff --git a/src/core/surface/call.c b/src/core/surface/call.c
index 20c41cbbd7..0a551ac47f 100644
--- a/src/core/surface/call.c
+++ b/src/core/surface/call.c
@@ -1230,7 +1230,8 @@ static void set_deadline_alarm(grpc_call *call, gpr_timespec deadline) {
   }
   GRPC_CALL_INTERNAL_REF(call, "alarm");
   call->have_alarm = 1;
-  grpc_alarm_init(&call->alarm, deadline, call_alarm, call, gpr_now());
+  grpc_alarm_init(&call->alarm, deadline, call_alarm, call,
+                  gpr_now(GPR_CLOCK_REALTIME));
 }
 
 /* we offset status by a small amount when storing it into transport metadata
diff --git a/src/core/surface/completion_queue.c b/src/core/surface/completion_queue.c
index 2ff47eb107..c67f75fc5c 100644
--- a/src/core/surface/completion_queue.c
+++ b/src/core/surface/completion_queue.c
@@ -260,8 +260,8 @@ grpc_pollset *grpc_cq_pollset(grpc_completion_queue *cc) {
 void grpc_cq_hack_spin_pollset(grpc_completion_queue *cc) {
   gpr_mu_lock(GRPC_POLLSET_MU(&cc->pollset));
   grpc_pollset_kick(&cc->pollset);
-  grpc_pollset_work(&cc->pollset,
-                    gpr_time_add(gpr_now(), gpr_time_from_millis(100)));
+  grpc_pollset_work(&cc->pollset, gpr_time_add(gpr_now(GPR_CLOCK_REALTIME),
+                                               gpr_time_from_millis(100)));
   gpr_mu_unlock(GRPC_POLLSET_MU(&cc->pollset));
 }
 
diff --git a/src/core/surface/init.c b/src/core/surface/init.c
index 3847ded28c..04e27d30ac 100644
--- a/src/core/surface/init.c
+++ b/src/core/surface/init.c
@@ -35,6 +35,7 @@
 
 #include <grpc/census.h>
 #include <grpc/grpc.h>
+#include <grpc/support/time.h>
 #include "src/core/channel/channel_stack.h"
 #include "src/core/client_config/resolver_registry.h"
 #include "src/core/client_config/resolvers/dns_resolver.h"
@@ -64,6 +65,7 @@ void grpc_init(void) {
 
   gpr_mu_lock(&g_init_mu);
   if (++g_initializations == 1) {
+    gpr_time_init();
     grpc_resolver_registry_init("dns:///");
     grpc_register_resolver_type("dns", grpc_dns_resolver_factory_create());
 #ifdef GPR_POSIX_SOCKET
diff --git a/src/core/surface/server.c b/src/core/surface/server.c
index eed2320e45..4dc51bf031 100644
--- a/src/core/surface/server.c
+++ b/src/core/surface/server.c
@@ -488,9 +488,9 @@ static void maybe_finish_shutdown(grpc_server *server) {
   if (server->root_channel_data.next != &server->root_channel_data ||
       server->listeners_destroyed < num_listeners(server)) {
     if (gpr_time_cmp(
-            gpr_time_sub(gpr_now(), server->last_shutdown_message_time),
+            gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), server->last_shutdown_message_time),
             gpr_time_from_seconds(1)) >= 0) {
-      server->last_shutdown_message_time = gpr_now();
+      server->last_shutdown_message_time = gpr_now(GPR_CLOCK_REALTIME);
       gpr_log(GPR_DEBUG,
               "Waiting for %d channels and %d/%d listeners to be destroyed"
               " before shutting down server",
@@ -948,7 +948,7 @@ void grpc_server_shutdown_and_notify(grpc_server *server,
     return;
   }
 
-  server->last_shutdown_message_time = gpr_now();
+  server->last_shutdown_message_time = gpr_now(GPR_CLOCK_REALTIME);
 
   channel_broadcaster_init(server, &broadcaster);
 
diff --git a/src/core/transport/chttp2/parsing.c b/src/core/transport/chttp2/parsing.c
index 130167f830..9597395aab 100644
--- a/src/core/transport/chttp2/parsing.c
+++ b/src/core/transport/chttp2/parsing.c
@@ -205,7 +205,8 @@ void grpc_chttp2_publish_reads(
     }
     if (stream_parsing->saw_rst_stream) {
       stream_global->cancelled = 1;
-      stream_global->cancelled_status = grpc_chttp2_http2_error_to_grpc_status(stream_parsing->rst_stream_reason);
+      stream_global->cancelled_status = grpc_chttp2_http2_error_to_grpc_status(
+          stream_parsing->rst_stream_reason);
       if (stream_parsing->rst_stream_reason == GRPC_CHTTP2_NO_ERROR) {
         stream_global->published_cancelled = 1;
       }
@@ -599,7 +600,7 @@ static void on_header(void *tp, grpc_mdelem *md) {
     }
     grpc_chttp2_incoming_metadata_buffer_set_deadline(
         &stream_parsing->incoming_metadata,
-        gpr_time_add(gpr_now(), *cached_timeout));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), *cached_timeout));
     GRPC_MDELEM_UNREF(md);
   } else {
     grpc_chttp2_incoming_metadata_buffer_add(&stream_parsing->incoming_metadata,
diff --git a/src/core/transport/chttp2/stream_encoder.c b/src/core/transport/chttp2/stream_encoder.c
index 56ab82006a..d553d80085 100644
--- a/src/core/transport/chttp2/stream_encoder.c
+++ b/src/core/transport/chttp2/stream_encoder.c
@@ -437,7 +437,8 @@ static void deadline_enc(grpc_chttp2_hpack_compressor *c, gpr_timespec deadline,
                          framer_state *st) {
   char timeout_str[GRPC_CHTTP2_TIMEOUT_ENCODE_MIN_BUFSIZE];
   grpc_mdelem *mdelem;
-  grpc_chttp2_encode_timeout(gpr_time_sub(deadline, gpr_now()), timeout_str);
+  grpc_chttp2_encode_timeout(
+      gpr_time_sub(deadline, gpr_now(GPR_CLOCK_REALTIME)), timeout_str);
   mdelem = grpc_mdelem_from_metadata_strings(
       c->mdctx, GRPC_MDSTR_REF(c->timeout_key_str),
       grpc_mdstr_from_string(c->mdctx, timeout_str));
diff --git a/src/core/transport/metadata.c b/src/core/transport/metadata.c
index 71fcb2fc14..e95b7a21f9 100644
--- a/src/core/transport/metadata.c
+++ b/src/core/transport/metadata.c
@@ -184,7 +184,7 @@ grpc_mdctx *grpc_mdctx_create(void) {
   /* This seed is used to prevent remote connections from controlling hash table
    * collisions. It needs to be somewhat unpredictable to a remote connection.
    */
-  return grpc_mdctx_create_with_seed(gpr_now().tv_nsec);
+  return grpc_mdctx_create_with_seed(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
 }
 
 static void discard_metadata(grpc_mdctx *ctx) {
diff --git a/src/csharp/ext/grpc_csharp_ext.c b/src/csharp/ext/grpc_csharp_ext.c
index ec125db78b..a55cc9e247 100644
--- a/src/csharp/ext/grpc_csharp_ext.c
+++ b/src/csharp/ext/grpc_csharp_ext.c
@@ -379,7 +379,7 @@ grpcsharp_channel_args_destroy(grpc_channel_args *args) {
 
 /* Timespec */
 
-GPR_EXPORT gpr_timespec GPR_CALLTYPE gprsharp_now(void) { return gpr_now(); }
+GPR_EXPORT gpr_timespec GPR_CALLTYPE gprsharp_now(void) { return gpr_now(GPR_CLOCK_REALTIME); }
 
 GPR_EXPORT gpr_timespec GPR_CALLTYPE gprsharp_inf_future(void) {
   return gpr_inf_future;
diff --git a/src/php/ext/grpc/timeval.c b/src/php/ext/grpc/timeval.c
index 8a278d6760..ccf7f0f81a 100644
--- a/src/php/ext/grpc/timeval.c
+++ b/src/php/ext/grpc/timeval.c
@@ -208,7 +208,7 @@ PHP_METHOD(Timeval, similar) {
  * @return Timeval The current time
  */
 PHP_METHOD(Timeval, now) {
-  zval *now = grpc_php_wrap_timeval(gpr_now());
+  zval *now = grpc_php_wrap_timeval(gpr_now(GPR_CLOCK_REALTIME));
   RETURN_DESTROY_ZVAL(now);
 }
 
diff --git a/src/ruby/ext/grpc/rb_completion_queue.c b/src/ruby/ext/grpc/rb_completion_queue.c
index 8fb3949b3d..2d52d96dc8 100644
--- a/src/ruby/ext/grpc/rb_completion_queue.c
+++ b/src/ruby/ext/grpc/rb_completion_queue.c
@@ -91,7 +91,7 @@ static void grpc_rb_completion_queue_shutdown_drain(grpc_completion_queue *cq) {
    * - investigate further, this is probably another example of C-level cleanup
    * not working consistently in all cases.
    */
-  next_call.timeout = gpr_time_add(gpr_now(), gpr_time_from_micros(5e3));
+  next_call.timeout = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(5e3));
   do {
     rb_thread_call_without_gvl(grpc_rb_completion_queue_next_no_gil,
                                (void *)&next_call, NULL, NULL);