Re: dirty pages (Was: Re: [PATCH] Prevent large file writeback starvation)

[Mark Lord <lkml@rtr.ca>]

Andrew Morton wrote:
> Johannes Stezenbach <js@linuxtv.org> wrote:
>> On Mon, Feb 06, 2006, Andrew Morton wrote:
>>> Mark Lord <lkml@rtr.ca> wrote:
>>>> A simple test I do for this:
>>>>
>>>>  $ mkdir t
>>>>  $ cp /usr/src/*.bz2  t    (about 400-500MB worth of kernel tar files)
>>>>  In another window, I do this:
>>>>  $ while (sleep 1); do echo -n "`date`: "; grep Dirty /proc/meminfo; done
>>>>  And then watch the count get large, but take virtually forever
>>>>  to count back down to a "safe" value.
>>>>  Typing "sync" causes all the Dirty pages to immediately be flushed to disk,
>>>>  as expected.
...
>> I've been seeing something like this for some time, but kept
>> silent as I'm forced to use vmware on my Thinkpad T42p (1G RAM,
>> but CONFIG_NOHIGHMEM=y).
>> Sometimes 'sync' takes serveral seconds, even when the machine
>> had been idle for >15mins. I don't have laptop mode enabled.
>> so far I've not found a deterinistic way to reproduce this behaviour.
>>
>> Anyway, I temporarily deinstalled vmware (deleted the kernel
>> modules and rebooted; kernel is still tainted because of madwifi
>> if that matters).
>> The behaviour I see with vmware (long 'sync' time) doesn't seem
>> to happen without it so far ...

Mmm.. Okay, all of my machines normally have VMWare-WS installed on them,
so that might just be the culprit.

MMMmm... isn't there an option somewhere in VMWare for lazy-writeback
or something like that, intended to speed up use of snapshots and
suspend/resume of VMs..  Ah, here is its description:

"Workstation uses a memory trimming technique to return unused virtual
machine memory to the host machine for other uses. While trimming usually
has little impact on performance and may be needed in low memory situations,
the I/O caused by memory trimming can sometimes interfere with disk-oriented
workload performance in a guest."

"Workstation uses a page sharing technique to allow guest memory pages with
identical contents to be stored as a single copy-on-write page. Page sharing
decreases host memory usage, but consumes some system resources, potentially
including I/O bandwidth. You may want to avoid this overhead for guests for
which host memory is plentiful and I/O latency is important."

Mmm.. so the intent is to affect only VMWare itself, not the rest of the
system while VMWare is dormant.  I guess it's time to disable loading of
the VMWare modules and reboot.  Bye bye uptime!   Maybe I'll install
2.6.16-rc3 while I'm at it.

I'll follow-up with results in an hour or two.

Cheers