linux-kernel - Re: Commit 31a12666d8f0c22235297e1c1575f82061480029 slows down Berkeley DB

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Message-Id: <200902031311.02378.nickpiggin@yahoo.com.au>
Date:	Tue, 3 Feb 2009 13:11:01 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	npiggin@...e.de
Subject: Re: Commit 31a12666d8f0c22235297e1c1575f82061480029 slows down Berkeley DB

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?

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