* Linus Torvalds [off-list ref] wrote:

On Fri, 3 Aug 2007, Peter Zijlstra wrote:

quoted

These patches aim to improve balance_dirty_pages() and directly address three
issues:
  1) inter device starvation
  2) stacked device deadlocks
  3) inter process starvation

Ok, the patches certainly look pretty enough, and you fixed the only 
thing I complained about last time (naming), so as far as I'm 
concerned it's now just a matter of whether it *works* or not. I guess 
being in -mm will help somewhat, but it would be good to have people 
with several disks etc actively test this out.

There are positive reports in the never-ending "my system crawls like an 
XT when copying large files" bugzilla entry:

 http://bugzilla.kernel.org/show_bug.cgi?id=7372

 " vfs_cache_pressure=1
   TCQ   nr_requests
   8     128    not that bad
   1     128    snappiest configuration, almost no pauses
                (or unnoticable ones) "
 
 " 1) vfs_cache_pressure at 100, 2.6.21.5+per bdi throttling patch 
   Result is good, not as snappier as I'd want during a large copy but 
   still usable. No process seems stuck for agen, but there seems to be 
   some short (second or subsecond) moment where everything is stuck 
   (like if you run a top d 0.5, the screen is not updated on a regular
   basis).

   2) vfs_cache_pressure at 1, 2.6.21.5+per bdi throttling patch Result
   is at 2.6.17 level. It is the better combination since 2.6.17. "

 " 1) I've applied the patches posted by Peter Zijlstra in comment #76 
   to the 2.6.21-mm2 kernel to check if it removes the problem. My
   impression is that the problem is still there with those patches,
   although less visible then with the clean 2.6.21 kernel. "

so the whole problem area seems to be a "perfect storm" created by a 
combination of TCQ, IO scheduling and VM dirty handling weaknesses. Per 
device dirty throttling is a good step forward and it makes a very 
visible positive difference.

	Ingo

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