From: Johannes Weiner <hannes@cmpxchg.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Mel Gorman <mgorman@suse.de>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
kernel-team@fb.com
Subject: Re: [PATCH] mm: fadvise: avoid expensive remote LRU cache draining after FADV_DONTNEED
Date: Mon, 12 Dec 2016 10:55:52 -0500 [thread overview]
Message-ID: <20161212155552.GA7148@cmpxchg.org> (raw)
In-Reply-To: <5cc0eb6f-bede-a34a-522b-e30d06723ffa@suse.cz>
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%.
> > Try to avoid the remote call by flushing the local LRU cache before
> > even attempting to invalidate anything. It's a cheap operation, and
> > the local LRU cache is the most likely to hold any pre-LRU pages in
> > the specified fadvise range.
>
> Anyway it looks like things can't be worse after this patch, so...
>
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
Thanks!
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next prev parent reply other threads:[~2016-12-12 15:56 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-10 17:26 [PATCH] mm: fadvise: avoid expensive remote LRU cache draining after FADV_DONTNEED Johannes Weiner
2016-12-12 9:21 ` Vlastimil Babka
2016-12-12 15:55 ` Johannes Weiner [this message]
2016-12-13 12:32 ` Vlastimil Babka
2016-12-14 21:00 ` [PATCH v2] " Johannes Weiner
2016-12-15 4:09 ` Hillf Danton
2016-12-12 9:51 ` [PATCH] " Mel Gorman
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