Messages in this thread | | | Date | Wed, 26 Apr 2006 21:15:58 +0200 | From | Jens Axboe <> | Subject | Re: Lockless page cache test results |
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On Wed, Apr 26 2006, Linus Torvalds wrote: > > > On Wed, 26 Apr 2006, Andrew Morton wrote: > > > Jens Axboe <axboe@suse.de> wrote: > > > > > > 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. > > > > OK. That doesn't sound like something which a real application is likely > > to do ;) > > True, but on the other hand, it does kind of "distill" one (small) part of > something that real apps _are_ likely to do. > > The whole 'splice to /dev/null' part can be seen as totally irrelevant, > but at the same time a way to ignore all the other parts of normal page > cache usage (ie the other parts of page cache usage tend to be the "map it > into user space" or the actual "memcpy_to/from_user()" or the "TCP send" > part). > > The question, of course, is whether the part that remains (the actual page > lookup) is important enough to matter, once it is part of a bigger chain > in a real application. > > In other words, the splice() thing is just a way to isolate one part of a > chain that is usually much more involved, and micro-benchmark just that > one part.
Nick called it a find_get_page() micro benchmark, which is pretty might spot on. So naturally it shows the absolute best side of the lockless page cache, but that is also very interesting. The /dev/null output can just be seen as a "infinitely" fast output method, both from a throughput and light weight POV.
> It would be interesting to see where doing gang-lookup moves the target, > but on the other hand, with smaller files (and small files are still > common), gang lookup isn't going to help as much.
With a 16-page gang lookup in splice, the top profile for the 4-client case (which is now at 4GiB/sec instead of 3) are:
samples % symbol name 30396 36.7217 __do_page_cache_readahead 25843 31.2212 find_get_pages_contig 9699 11.7174 default_idle
Even disregarding that readahead contender that could probably be made a little more clever, we are still spending an awful lot of time in the page lookup. I didn't mention this before, but the get_page/put_page overhead is also a lot smaller with the lockless patches.
-- Jens Axboe
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