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    SubjectRe: Commit 31a12666d8f0c22235297e1c1575f82061480029 slows down Berkeley DB
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    On Tue, 2009-02-03 at 13:11 +1100, Nick Piggin wrote:
    > On Tuesday 03 February 2009 12:54:26 Zhang, Yanmin wrote:
    > > On Tue, 2009-02-03 at 12:24 +1100, Nick Piggin wrote:
    > > > On Friday 30 January 2009 12:23:15 Jan Kara wrote:
    > > > > Hi,
    > > > >
    > > > > today I found that commit 31a12666d8f0c22235297e1c1575f82061480029
    > > > > (mm: write_cache_pages cyclic fix) slows down operations over Berkeley
    > > > > DB. Without this "fix", I can add 100k entries in about 5 minutes 30s,
    > > > > with that change it takes about 20 minutes.
    > > > > What is IMO happening is that previously we scanned to the end of
    > > > > file, we left writeback_index at the end of file and went to write next
    > > > > file. With the fix, we wrap around (seek) and after writing some more
    > > > > we go to next file (seek again).
    > >
    > > We also found this commit causes about 40~50% regression with iozone
    > > mmap-rand-write. #iozone -B -r 4k -s 64k -s 512m -s 1200m
    > >
    > > My machine has 8GB memory.
    >
    > Ah, thanks. Yes BDB I believe is basically just doing an mmap-rand-write,
    > so maybe this is a good test case.
    >
    > The interesting thing is why is this causing such a slowdown. If there is
    > only a single main file active in the workload, then I don't see why this
    > patch should make such a big difference. In either case, wouldn't pdflush
    > come back and just start writing out from the start of the file anyway?

    Perhaps the difference is that without the patch, pdflush will return
    after running congestion_wait()? This would give bdb and iozone a
    chance to fill in more pages, and increases the chances we'll do
    sequential IO.

    -chris




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