Messages in this thread | | | Subject | Re: [PATCH] mm: fadvise: avoid expensive remote LRU cache draining after FADV_DONTNEED | From | Vlastimil Babka <> | Date | Mon, 12 Dec 2016 10:21:24 +0100 |
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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?
> 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>
> --- > mm/fadvise.c | 15 ++++++++++++++- > 1 file changed, 14 insertions(+), 1 deletion(-) > > diff --git a/mm/fadvise.c b/mm/fadvise.c > index 6c707bfe02fd..a43013112581 100644 > --- a/mm/fadvise.c > +++ b/mm/fadvise.c > @@ -139,7 +139,20 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice) > } > > if (end_index >= start_index) { > - unsigned long count = invalidate_mapping_pages(mapping, > + unsigned long count; > + > + /* > + * It's common to FADV_DONTNEED right after > + * the read or write that instantiates the > + * pages, in which case there will be some > + * sitting on the local LRU cache. Try to > + * avoid the expensive remote drain and the > + * second cache tree walk below by flushing > + * them out right away. > + */ > + lru_add_drain(); > + > + count = invalidate_mapping_pages(mapping, > start_index, end_index); > > /* >
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