Re: [PATCH] mm: fadvise: avoid expensive remote LRU cache draining after FADV_DONTNEED

[Vlastimil Babka]

On 12/12/2016 04:55 PM, Johannes Weiner wrote:
> On Mon, Dec 12, 2016 at 10:21:24AM +0100, Vlastimil Babka wrote:
> > On 12/10/2016 06:26 PM, Johannes Weiner wrote:
> > > When FADV_DONTNEED cannot drop all pages in the range, it observes
> > > that some pages might still be on per-cpu LRU caches after recent
> > > instantiation and so initiates remote calls to all CPUs to flush their
> > > local caches. However, in most cases, the fadvise happens from the
> > > same context that instantiated the pages, and any pre-LRU pages in the
> > > specified range are most likely sitting on the local CPU's LRU cache,
> > > and so in many cases this results in unnecessary remote calls, which,
> > > in a loaded system, can hold up the fadvise() call significantly.
> >
> > Got any numbers for this part?
>
> I didn't record it in the extreme case we observed, unfortunately. We
> had a slow-to-respond system and noticed it spending seconds in
> lru_add_drain_all() after fadvise calls, and this patch came out of
> thinking about the code and how we commonly call FADV_DONTNEED.
>
> FWIW, I wrote a silly directory tree walker/searcher that recurses
> through /usr to read and FADV_DONTNEED each file it finds. On a 2
> socket 40 ht machine, over 1% is spent in lru_add_drain_all(). With
> the patch, that cost is gone; the local drain cost shows at 0.09%.

Thanks, worth adding to changelog :)

Vlastimil