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Remove use of mmap for random reads

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Summary:
Reads via mmap on concurrent workloads are much slower than pread.
For example on a 24-core server with storage that can do 100k IOPS or more
I can get no more than 10k IOPS with mmap reads and 32+ threads.

Test Plan: db_bench benchmarks

Reviewers: dhruba, heyongqiang

Reviewed By: heyongqiang

Differential Revision: https://reviews.facebook.net/D5433
This commit is contained in:
Mark Callaghan 2012-09-14 12:35:02 -07:00
parent fa29f82548
commit 33323f2111
1 changed files with 2 additions and 13 deletions
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15
util/env_posix.cc
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@ -386,24 +386,13 @@ class PosixEnv : public Env {
virtual Status NewRandomAccessFile(const std::string& fname, virtual Status NewRandomAccessFile(const std::string& fname,
RandomAccessFile** result) { RandomAccessFile** result) {
// Use of mmap for random reads has been removed because it
// kills performance when storage is fast.
*result = NULL; *result = NULL;
Status s; Status s;
int fd = open(fname.c_str(), O_RDONLY); int fd = open(fname.c_str(), O_RDONLY);
if (fd < 0) { if (fd < 0) {
s = IOError(fname, errno); s = IOError(fname, errno);
} else if (useOsBuffer && sizeof(void*) >= 8) {
// Use mmap when virtual address-space is plentiful.
uint64_t size;
s = GetFileSize(fname, &size);
if (s.ok()) {
void* base = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
if (base != MAP_FAILED) {
*result = new PosixMmapReadableFile(fname, base, size);
} else {
s = IOError(fname, errno);
}
}
close(fd);
} else { } else {
*result = new PosixRandomAccessFile(fname, fd); *result = new PosixRandomAccessFile(fname, fd);
} }