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SubjectRe: [PATCH v4 00/11] Performance fixes for 9p filesystem
Glad to hear bugs disappeared.  writeback having a different
performance than mmap is confusing as they should be equivalent.

The huge blocksize on your dd is an interesting choice -- it will
completely get rid of any impact of readahead. To see impact of
readahead, choose a blocksize of
less than msize (like 4k) to actually see the perf of readahead. The
mmap degradation is likely due to stricter coherence (open-to-close
consistency means we wait on writeout), but I'd probably need to go in
and trace to verify (which probably isn't a bad idea overall).
probably a similar situation for loose and writeback. Essentially,
before close consistency it didn't have to wait for the final write to
complete before it returns so you see a faster time (even though data
wasn't actually written all the way through so you aren't measuring
the last little bit of the write (which can be quite large of a big

I'm going to take a pass through tomorrow making some fixups that
Dominiquee suggested and trying to reproduce/fix the fscache problems.


On Sun, Feb 19, 2023 at 3:36 PM Christian Schoenebeck
<> wrote:
> On Saturday, February 18, 2023 1:33:12 AM CET Eric Van Hensbergen wrote:
> > This is the fourth version of a patch series which adds a number
> > of features to improve read/write performance in the 9p filesystem.
> > Mostly it focuses on fixing caching to help utilize the recently
> > increased MSIZE limits and also fixes some problematic behavior
> > within the writeback code.
> >
> > All together, these show roughly 10x speed increases on simple
> > file transfers over no caching for readahead mode. Future patch
> > sets will improve cache consistency and directory caching, which
> > should benefit loose mode.
> >
> > This iteration of the patch incorporates an important fix for
> > writeback which uses a stronger mechanism to flush writeback on
> > close of files and addresses observed bugs in previous versions of
> > the patch for writeback, mmap, and loose cache modes.
> >
> > These patches are also available on github:
> >
> > and on
> >
> >
> > Tested against qemu, cpu, and diod with fsx, dbench, and postmark
> > in every caching mode.
> >
> > I'm gonna definitely submit the first couple patches as they are
> > fairly harmless - but would like to submit the whole series to the
> > upcoming merge window. Would appreciate reviews.
> I tested this version thoroughly today (msize=512k in all tests). Good news
> first: the previous problems of v3 are gone. Great! But I'm still trying to
> make sense of the performance numbers I get with these patches.
> So when doing some compilations with 9p, performance of mmap, writeback and
> readahead are basically all the same, and only loose being 6x faster than the
> other cache modes. Expected performance results? No errors at least. Good!
> Then I tested simple linear file I/O. First linear writing a 12GB file
> (time dd if=/dev/zero bs=1G count=12):
> writeback 3m10s [this series - v4]
> readahead 0m11s [this series - v4]
> mmap 0m11s [this series - v4]
> mmap 0m11s [master]
> loose 2m50s [this series - v4]
> loose 2m19s [master]
> That's a bit surprising. Why is loose and writeback slower?
> Next linear reading a 12GB file
> (time cat > /dev/null):
> writeback 0m24s [this series - v4]
> readahead 0m25s [this series - v4]
> mmap 0m25s [this series - v4]
> mmap 0m9s [master]
> loose 0m24s [this series - v4]
> loose 0m24s [master]
> mmap degredation sticks out here, and no improvement with the other modes?
> I always performed a guest reboot between each run BTW.

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