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Resolve race when getting approximate-memory-usage property

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The write operations in the table happens without holding the mutex
lock, but concurrent writes are avoided using "writers_" queue.
The Arena::MemoryUsage could access the blocks when write happens.
So, the memory usage is cached in atomic word and can be loaded
from any thread safely.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=107573379
This commit is contained in:
ssid 2015-11-11 03:54:18 -08:00 committed by Chris Mumford
parent 3c9ff3c691
commit 706b7f8d43
3 changed files with 9 additions and 12 deletions
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5
db/memtable.h
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@ -36,10 +36,7 @@ class MemTable {
} }
// Returns an estimate of the number of bytes of data in use by this // Returns an estimate of the number of bytes of data in use by this
// data structure. // data structure. It is safe to call when MemTable is being modified.
//
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
size_t ApproximateMemoryUsage(); size_t ApproximateMemoryUsage();
// Return an iterator that yields the contents of the memtable. // Return an iterator that yields the contents of the memtable.

6
util/arena.cc
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@ -9,8 +9,7 @@ namespace leveldb {
static const int kBlockSize = 4096; static const int kBlockSize = 4096;
Arena::Arena() { Arena::Arena() : memory_usage_(0) {
blocks_memory_ = 0;
alloc_ptr_ = NULL; // First allocation will allocate a block alloc_ptr_ = NULL; // First allocation will allocate a block
alloc_bytes_remaining_ = 0; alloc_bytes_remaining_ = 0;
} }
@ -60,8 +59,9 @@ char* Arena::AllocateAligned(size_t bytes) {
char* Arena::AllocateNewBlock(size_t block_bytes) { char* Arena::AllocateNewBlock(size_t block_bytes) {
char* result = new char[block_bytes]; char* result = new char[block_bytes];
blocks_memory_ += block_bytes;
blocks_.push_back(result); blocks_.push_back(result);
memory_usage_.NoBarrier_Store(
reinterpret_cast<void*>(MemoryUsage() + block_bytes + sizeof(char*)));
return result; return result;
} }

10
util/arena.h
View File

@ -9,6 +9,7 @@
#include <assert.h> #include <assert.h>
#include <stddef.h> #include <stddef.h>
#include <stdint.h> #include <stdint.h>
#include "port/port.h"
namespace leveldb { namespace leveldb {
@ -24,10 +25,9 @@ class Arena {
char* AllocateAligned(size_t bytes); char* AllocateAligned(size_t bytes);
// Returns an estimate of the total memory usage of data allocated // Returns an estimate of the total memory usage of data allocated
// by the arena (including space allocated but not yet used for user // by the arena.
// allocations).
size_t MemoryUsage() const { size_t MemoryUsage() const {
return blocks_memory_ + blocks_.capacity() * sizeof(char*); return reinterpret_cast<uintptr_t>(memory_usage_.NoBarrier_Load());
} }
private: private:
@ -41,8 +41,8 @@ class Arena {
// Array of new[] allocated memory blocks // Array of new[] allocated memory blocks
std::vector<char*> blocks_; std::vector<char*> blocks_;
// Bytes of memory in blocks allocated so far // Total memory usage of the arena.
size_t blocks_memory_; port::AtomicPointer memory_usage_;
// No copying allowed // No copying allowed
Arena(const Arena&); Arena(const Arena&);