Re: [PATCH]: VM 6/8 page_referenced(): move dirty

[Nikita Danilov <nikita@clusterfs.com>]

Andrew Morton writes:
 > Nikita Danilov <nikita@clusterfs.com> wrote:
 > >
 > >   > 
 > >   > I can envision workloads (such as mmap 80% of memory and continuously dirty
 > >   > it) which would end up performing continuous I/O with this patch.
 > > 
 > >  Below is a version that tries to move dirtiness to the struct page only
 > >  if we are really going to deactivate the page. In your scenario above,
 > >  continuously dirty pages will be on the active list, so it should be
 > >  okay.
 > 
 > OK, well it'll now increase the amount of I/O by a smaller amount.  Trade
 > that off against possibly improved I/O patterns.  But how do we know that
 > all this is a net gain?

By looking at the (micro-) benchmarking results:

2.6.12-rc2:

before-patch page_referenced-move-dirty

        45.8  32.3
       204.3  93.2
       194.8  89.5
       194.9  89.9
       197.7  92.1
       195.0  90.2
       199.4  89.5
       196.3  89.2

Numbers are seconds it took to dirty 1G of mmapped file on box with
mem=64m (first row is different, because file was just created, and
consists of one huge hole).

Also I have seen that this patch improves oom-resistance on a box with
small memory, all of which is dirtied through mmap. One possible
explanation is that blk_congestion_wait() in try_to_free_pages() doesn't
throttle scanner enough.

 > 
 > >   > 
 > >   > IOW: I'm gonna drop this one like it's made of lead!
 > > 
 > >  Let's decrease atomic number by 3.
 > 
 > Still heavy.
 > 

Minus one more.

Nikita.
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