Re: Swappiness vs. mmap() and interactive response

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 29 Apr 2009 14:51:07 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> wrote:

> Hi
> 
> > 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.
> 
> firstly, I'd like to report my reproduce test result.
> 
> test environment: no lvm, copy ext3 to ext3 (not mv), no change swappiness, 
>                   CFQ is used, userland is Fedora10, mmotm(2.6.30-rc1 + mm patch),
>                   CPU opteronx4, mem 4G
> 
> mouse move lag:               not happend
> window move lag:              not happend
> Mapped page decrease rapidly: not happend (I guess, these page stay in 
>                                           active list on my system)
> page fault large latency:     happend (latencytop display >200ms)

hm.  The last two observations appear to be inconsistent.

Elladan, have you checked to see whether the Mapped: number in
/proc/meminfo is decreasing?

> 
> Then, I don't doubt vm replacement logic now.
> but I need more investigate.
> I plan to try following thing today and tommorow.
> 
>  - XFS
>  - LVM
>  - another io scheduler (thanks Ted, good view point)
>  - Rik's new patch

It's not clear that we know what's happening yet, is it?  It's such a
gross problem that you'd think that even our testing would have found
it by now :(

Elladan, do you know if earlier kernels (2.6.26 or thereabouts) had
this severe a problem?

(notes that we _still_ haven't unbusted prev_priority)