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Correct the logic of MemTable::ShouldFlushNow().

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Summary:
Memtable will now be forced to flush if the one of the following
conditions is met:
1. Already allocated more than write_buffer_size + 60% arena block size.
   (the overflowing condition)
2. Unable to safely allocate one more arena block without hitting the
   overflowing condition AND the unused allocated memory < 25% arena
   block size.

Test Plan: make all check

Reviewers: sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16893
This commit is contained in:
Yueh-Hsuan Chiang 2014-03-14 14:01:45 -07:00
parent 9b8a2b52d4
commit a5fafd4f46
1 changed files with 8 additions and 6 deletions
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14
db/memtable.cc
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@ -78,21 +78,23 @@ bool MemTable::ShouldFlushNow() const {
auto allocated_memory =
table_->ApproximateMemoryUsage() + arena_.MemoryAllocatedBytes();
if (allocated_memory + kArenaBlockSize * kAllowOverAllocationRatio <
kWriteBufferSize) {
// if we can still allocate one more block without exceeding the
// over-allocation ratio, then we should not flush.
if (allocated_memory + kArenaBlockSize <
kWriteBufferSize + kArenaBlockSize * kAllowOverAllocationRatio) {
return false;
}
// if user keeps adding entries that exceeds kWriteBufferSize, we need to
// flush
// earlier even though we still have much available memory left.
if (allocated_memory > kWriteBufferSize * (1 + kAllowOverAllocationRatio)) {
// flush earlier even though we still have much available memory left.
if (allocated_memory >
kWriteBufferSize + kArenaBlockSize * kAllowOverAllocationRatio) {
return true;
}
// In this code path, Arena has already allocated its "last block", which
// means the total allocatedmemory size is either:
// (1) "moderately" over allocated the memory (no more than `0.4 * arena
// (1) "moderately" over allocated the memory (no more than `0.6 * arena
// block size`. Or,
// (2) the allocated memory is less than write buffer size, but we'll stop
// here since if we allocate a new arena block, we'll over allocate too much