Re: Lockless page cache test results

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On Wed, Apr 26 2006, Andrew Morton wrote:
> Jens Axboe <[email protected]> wrote:
> >
> > Running a splice benchmark on a 4-way IPF box, I decided to give the
> >  lockless page cache patches from Nick a spin. I've attached the results
> >  as a png, it pretty much speaks for itself.
> It does.
> What does the test do?
> In particular, does it cause the kernel to take tree_lock once per
> page, or once per batch-of-pages?

Once per page, it's basically exercising the generic_file_splice_read()
path. Basically X number of "clients" open the same file, and fill those
pages into a pipe using splice. The output end of the pipe is then
spliced to /dev/null to toss it away again. The top of the 4-client
vanilla run profile looks like this:

samples  %        symbol name
65328    47.8972  find_get_page

Basically the machine is fully pegged, about 7% idle time.

We can speedup the lookups with find_get_pages(). The test does 64k max,
so with luck we should be able to pull 16 pages in at the time. I'll try
and run such a test. But boy I wish find_get_pages_contig() was there
for that. I think I'd prefer adding that instead of coding that logic in
splice, it can get a little tricky.

Jens Axboe

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