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/*
* Copyright 2017 Google Inc.
*
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
*/
#ifndef SkTaskGroup2D_DEFINED
#define SkTaskGroup2D_DEFINED
#include "SkTaskGroup.h"
#include <mutex>
#include <vector>
// The interface for doing work on a 2D grid with possible initialization on columns.
class SkWorkKernel2D {
public:
// Return false iff the column needs initialization and such initialization is not finished yet.
virtual bool work2D(int row, int column, int thread) = 0;
// Return false if no initialization is done for this colum (e.g., it's already initialized; or
// maybe some other thread is initializing the column).
virtual bool initColumn(int column, int thread) = 0;
virtual ~SkWorkKernel2D() {}
};
// A 2D grid (height rows x width columns) of tasks to be executed on a given executor with
// threadCnt number of threads.
//
// The height (number of rows) is fixed. The width (number of columns) may be dynamically expanded.
//
// The task on row i and column j is abstracted as work2D(i, j, t). Parameter t is the thread id and
// it shouldn't affect the work to be done. It's only used to allow some variables that are not
// thread safe and should be used exclusively by one thread (e.g., thread allocators). We guarantee
// that the task on the same row will be executed in order (i.e., work2D(1, 1, t) is guaranteed to
// finish before calling work2D(1, 2, t)). Tasks in different rows can happen in any order.
//
// There are also width number of init calls, one per column. work2D(i, j, t) may return false if
// column j requires initialization but it's not initialized yet. In that case, a thread t needs to
// call initColumn(j, t) once to unblock all rows that depend on the initialization of column j.
// (Again, t shouldn't affect the init work to be done; it's just for some non-thread-safe
// variables). The init calls have no order requirement so we can call them in any order.
//
// Multiple therads may try to init the same column j at the same time. InitFn is expected to handle
// this gracefully (e.g., let only one thread do the init and return immediately for other threads).
class SkTaskGroup2D {
public:
SkTaskGroup2D(SkWorkKernel2D* kernel, int height, SkExecutor* executor, int threadCnt)
: fKernel(kernel), fHeight(height), fThreadCnt(threadCnt), fIsFinishing(false)
, fWidth(0), fThreadsGroup(new SkTaskGroup(*executor)) {}
virtual ~SkTaskGroup2D() {}
virtual void addColumn(); // Add a new column of tasks.
void start(); // start threads to execute tasks
void finish(); // wait and finish all tasks (no more tasks can be added after calling this)
SK_ALWAYS_INLINE bool isFinishing() const {
return fIsFinishing.load(std::memory_order_relaxed);
}
protected:
static constexpr int MAX_CACHE_LINE = 64;
// Finish all tasks on the threadId and then return.
virtual void work(int threadId) = 0;
// Initialize a column that needs to be initialized. The parameter initCol is not thread safe
// and should only be exclusively accessed by the working thread which will modify it to the
// column that may need to be initialized next.
void initAnUninitializedColumn(int& initCol, int threadId) {
bool didSomeInit = false;
while (initCol < fWidth && !didSomeInit) {
didSomeInit = fKernel->initColumn(initCol++, threadId);
}
}
SkWorkKernel2D* fKernel;
const int fHeight;
const int fThreadCnt;
std::atomic<bool> fIsFinishing;
std::atomic<int> fWidth;
std::unique_ptr<SkTaskGroup> fThreadsGroup;
};
// A simple spinning task group that assumes height equals threadCnt.
class SkSpinningTaskGroup2D final : public SkTaskGroup2D {
public:
SkSpinningTaskGroup2D(SkWorkKernel2D* kernel, int h, SkExecutor* x, int t)
: SkTaskGroup2D(kernel, h, x, t) {
SkASSERT(h == t); // height must be equal to threadCnt
}
protected:
void work(int threadId) override;
};
class SkFlexibleTaskGroup2D final : public SkTaskGroup2D {
public:
SkFlexibleTaskGroup2D(SkWorkKernel2D* kernel, int h, SkExecutor* x, int t)
: SkTaskGroup2D(kernel, h, x, t), fRowData(h) {}
protected:
void work(int threadId) override;
private:
// alignas(MAX_CACHE_LINE) to avoid false sharing by cache lines
struct alignas(MAX_CACHE_LINE) RowData {
RowData() : fNextColumn(0) {}
int fNextColumn; // next column index to work
std::mutex fMutex; // the mutex for the thread to acquire
};
std::vector<RowData> fRowData;
};
#endif//SkTaskGroup2D_DEFINED
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