Re: Performance regression in scsi sequential throughput (iozone) due to "e084b - page-allocator: preserve PFN ordering when __GFP_COLD is set"

[Nick Piggin <npiggin@suse.de>]

On Fri, Feb 19, 2010 at 03:19:34PM +0000, Mel Gorman wrote:
> On Fri, Feb 19, 2010 at 12:19:27PM +0100, Christian Ehrhardt wrote:
> > Eventually it might come down to a discussion of allocation priorities and
> > we might even keep them as is and accept this issue - I still would prefer
> > a good second chance implementation, other page cache allocation flags or
> > something else that explicitly solves this issue.
> >
> 
> In that line, the patch that replaced congestion_wait() with a waitqueue
> makes some sense.
> 
> > Mel's patch that replaces congestion_wait with a wait for the zone watermarks
> > becoming available again is definitely a step in the right direction and
> > should go into upstream and the long term support branches.
> 
> I'll need to do a number of tests before I can move that upstream but I
> don't think it's a merge candidate. Unfortunately, I'll be offline for a
> week starting tomorrow so I won't be able to do the testing.
> 
> When I get back, I'll revisit those patches with the view to pushing
> them upstream. I hate to treat symptoms here without knowing the
> underlying problem but this has been spinning in circles for ages with
> little forward progress :(

The zone pressure waitqueue patch makes sense. We may even want to make
it more strictly FIFO (eg. check upfront if there are waiters on the
queue before allocating a page, and if yes then add ourself to the back
of the waitqueue). And also possibly even look at doing the wakeups in
the page-freeing path. Although that might start adding too much
overhead, so it's quite possible your sloppy-but-lighter timeout
approach is preferable.