Re: [PATCH] mm/filemap.c: unconditionally call mark_page_accessed

[Xiaoning Ding]

Dave Kleikamp wrote:
> On Wed, 2007-03-14 at 22:33 +0100, Andreas Mohr wrote:
> > Hi,
> >
> > On Wed, Mar 14, 2007 at 03:55:41PM -0500, Dave Kleikamp wrote:
> > > On Wed, 2007-03-14 at 15:58 -0400, Ashif Harji wrote:
> > > > This patch unconditionally calls mark_page_accessed to prevent pages, 
> > > > especially for small files, from being evicted from the page cache despite 
> > > > frequent access.
> > > I guess the downside to this is if a reader is reading a large file, or
> > > several files, sequentially with a small read size (smaller than
> > > PAGE_SIZE), the pages will be marked active after just one read pass.
> > > My gut says the benefits of this patch outweigh the cost.  I would
> > > expect real-world backup apps, etc. to read at least PAGE_SIZE.
> > I also think that the patch is somewhat problematic, since the original
> > intention seems to have been a reduction of the number of (expensive?)
> > mark_page_accessed() calls,
>
> mark_page_accessed() isn't expensive.  If called repeatedly, starting
> with the third call, it will check two page flags and return.  The only
> real expense is that the page appears busier than it may be and will be
> retained in memory longer than it should.
If we allow mark_page_accessed() called multiple times for a single page,
a scan of large file with small-size reads would flush the buffer cache.
mark_page_accessed() also requests lru_lock when moving page from
inactive_list to active_list. It may also increase lock contention.

> > but this of course falls flat on its face in case
> > of permanent single-page accesses or accesses with progressing but very small
> > read size (single-byte reads or so), since the cached page content will expire
> > eventually due to lack of mark_page_accessed() updates; thus this patch
> > decided to call mark_page_accessed() unconditionally which may be a large
> > performance penalty for subsequent tiny-sized reads.
>
> Any application doing many tiny-sized reads isn't exactly asking for
> great performance.
>
> > I've been thinking hard how to avoid the mark_page_accessed() starvation in
> > case of a fixed, (almost) non-changing access state, but this seems hard since
> > it'd seem we need some kind of state management here to figure out good
> > intervals of when to call mark_page_accessed() *again* for this page. E.g.
> > despite non-changing access patterns you could still call mark_page_accessed()
> > every 32 calls or so to avoid expiry, but this would need extra helper
> > variables.
> >
> > A rather ugly way to do it may be to abuse ra.cache_hit or ra.mmap_hit content
> > with a
> > 	if ((prev_index != index) || (ra.cache_hit % 32 == 0))
> > 		mark_page_accessed(page);
> > This assumes that ra.cache_hit gets incremented for every access (haven't
> > checked whether this is the case).
> > That way (combined with an enhanced comment properly explaining the dilemma)
> > you would avoid most mark_page_accessed() invocations of subsequent same-page reads
> > but still do page status updates from time to time to avoid page deprecation.
> >
> > Does anyone think this would be acceptable? Any better idea?
>
> I wouldn't go looking for anything more complicated than Ashif's patch,
> unless testing shows it to be harmful in some realistic workload.
>
> > Andreas Mohr
> >
> > P.S.: since I'm not too familiar with this area I could be rather wrong after all...
>
> I could be missing something as well.  :-)
>
> Shaggy

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