Re: [PATCH v9 07/20] mm/lru: introduce TestClearPageLRU

Andrew Morton
2020-03-04T00:46:59Z

On Tue, 3 Mar 2020 12:11:34 +0800 Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org> wrote:

>
>
> 在 2020/3/3 上午6:11, Andrew Morton 写道:
> >> - if (PageLRU(page)) {
> >> + if (TestClearPageLRU(page)) {
> >> lruvec = mem_cgroup_page_lruvec(page, pgdat);
> >> - ClearPageLRU(page);
> >> del_page_from_lru_list(page, lruvec, page_lru(page));
> >> } else
> >
> > The code will now get exclusive access of the page->flags cacheline and
> > will dirty that cacheline, even for !PageLRU() pages. What is the
> > performance impact of this?
> >
>
> Hi Andrew,
>
> Thanks a lot for comments!
>
> I was tested the whole patchset with fengguang's case-lru-file-readtwice
> https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git/
> which is most sensitive case on PageLRU I found. There are no clear performance
> drop.
>
> On this single patch, I just test the same case again, there is still no perf
> drop. some data is here on my 96 threads machine:
>
> no lock_dep w lock_dep and few other debug option
> w this patch, 50.96MB/s 32.93MB/s
> w/o this patch, 50.50MB/s 33.53MB/s
>
>

Well, any difference would be small and the numbers did get a bit
lower, albeit probably within the margin of error.

But you know, if someone were to send a patch which micro-optimized
some code by replacing 'TestClearXXX()' with `if PageXXX()
ClearPageXXX()', I would happily merge it!

Is this change essential to the overall patchset? If not, I'd be
inclined to skip it?