Re: Swappiness vs. mmap() and interactive response

[Theodore Tso]

On Tue, Apr 28, 2009 at 05:09:16PM +0800, Wu Fengguang wrote:
> The semi-drop-behind is a great idea for the desktop - to put just
> accessed pages to end of LRU. However I'm still afraid it vastly
> changes the caching behavior and wont work well as expected in server
> workloads - shall we verify this?
> 
> Back to this big-cp-hurts-responsibility issue. Background write
> requests can easily pass the io scheduler's obstacles and fill up
> the disk queue. Now every read request will have to wait 10+ writes
> - leading to 10x slow down of major page faults.
> 
> I reach this conclusion based on recent CFQ code reviews. Will bring up
> a queue depth limiting patch for more exercises..

We can muck with the I/O scheduler, but another thing to consider is
whether the VM should be more aggressively throttling writes in this
case; it sounds like the big cp in this case may be dirtying pages so
aggressively that it's driving other (more useful) pages out of the
page cache --- if the target disk is slower than the source disk (for
example, backing up a SATA primary disk to a USB-attached backup disk)
no amount of drop-behind is going to help the situation.

So that leaves three areas for exploration:

* Write-throttling
* Drop-behind
* background writes pushing aside foreground reads

Hmm, note that although the original bug reporter is running Ubuntu
Jaunty, and hence 2.6.28, this problem is going to get *worse* with
2.6.30, since we have the ext3 data=ordered latency fixes which will
write out the any journal activity, and worse, any synchornous commits
(i.e., caused by fsync) will force out all of the dirty pages with
WRITE_SYNC priority.  So with a heavy load, I suspect this is going to
be more of a VM issue, and especially figuring out how to tune more
aggressive write-throttling may be key here.

					- Ted
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