No subject

not throttle/block until reclaim priority has got pretty high. That
means at low priority reclaim, it *skips pages under writeback*.
However, if it comes across a dirty page, it will trigger writeback
of the page.

Now call me crazy, but if we've already got a large number of pages
under writeback, why would we want to *start more IO* when clearly
the system is taking care of cleaning pages already and all we have
to do is wait for a short while to get clean pages ready for
reclaim?

Indeed, I added this quick hack to prevent the VM from doing
writeback via pageout until after it starts blocking on writeback
pages:

@@ -825,6 +825,8 @@ static unsigned long shrink_page_list(struct list_head *page_l
 		if (PageDirty(page)) {
 			nr_dirty++;
 
+			if (!(sc->reclaim_mode & RECLAIM_MODE_SYNC))
+				goto keep_locked;
 			if (references == PAGEREF_RECLAIM_CLEAN)
 				goto keep_locked;
 			if (!may_enter_fs)

IOWs, we don't write pages from kswapd unless there is no IO
writeback going on at all (waited on all the writeback pages or none
exist) and there are dirty pages on the LRU.

This doesn't completely stop the IO collapse, (looks like foreground
throttling is the other cause, which IO-less write throttling fixes)
but the collapse was significantly reduced in duration and intensity
by removing kswapd writeback. In fact, the IO rate only dropped to
~60MB/s instead of 30MB/s, and the improvement is easily measured by
the runtime of the test:

			run 1	run 2	run 3
3.0-rc5-vanilla		135s	137s	138s
3.0-rc5-patched		117s	115s	115s

That's a pretty massive improvement for a 2-line patch. ;) I expect
the IO-less write throttling patchset will further improve this.

FWIW, the nr_vmscan_write values changed like this:

			run 1	run 2	run 3
3.0-rc5-vanilla		6751	6893	6465
3.0-rc5-patched		0	0	0

These results support my argument that memory reclaim should not be
doing dirty page writeback at all - defering writeback to the
writeback infrastructure and just waiting for it to complete
appropriately is the Right Thing To Do. i.e. IO-less memory reclaim
works better than the current code for the same reason IO-less write
throttling works better than the current code....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com