Re: [PATCH]: VM 3/8 PG_skipped

[Hirokazu Takahashi <taka@valinux.co.jp>]

Hi,

> >  Don't call ->writepage from VM scanner when page is met for the first time
> >  during scan.
> > 
> >  New page flag PG_skipped is used for this. This flag is TestSet-ed just
> >  before calling ->writepage and is cleaned when page enters inactive
> >  list.
> > 
> >  One can see this as "second chance" algorithm for the dirty pages on the
> >  inactive list.
> > 
> >  BSD does the same: src/sys/vm/vm_pageout.c:vm_pageout_scan(),
> >  PG_WINATCFLS flag.
> > 
> >  Reason behind this is that ->writepages() will perform more efficient writeout
> >  than ->writepage(). Skipping of page can be conditioned on zone->pressure.
> > 
> >  On the other hand, avoiding ->writepage() increases amount of scanning
> >  performed by kswapd.
> 
> I worry that this will cause boxes to go oom all over the place, due to the
> longer scans which are encountered prior to pages being reclaimed.
> 
> We could of course increase the "oh crap, we've scanned too much"
> threshold.  We probably need to do that anyway - I shrunk it by heaps early
> in 2.5 just as a "let's see who complains" experiment.
> 
> Writeout off the LRU should be a rare case.  We should have instrumentation
> for that, but we don't.

IMHO, I don't think it's always true.
Would you please consider mmap'ed pages, which may not be written out
by pdflush. It might leave many unexpected dirty pages to the swapcode.

I feel it would be better to let pdflush do this things, though.

> My gut feel with this patch is to run away in terror, frankly.


Thanks,
Hirokazu Takahashi.
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"aart@kvack.org"> aart@kvack.org </a>