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SubjectRe: ~500 megs cached yet 2.6.5 goes into swap hell
On Wed, Apr 28, 2004 at 08:19:24PM -0700, Andrew Morton wrote:
> What you discuss above is just an implementation detail. Forget it. What
> are the requirements? Thus far I've seen
>
> a) updatedb causes cache reclaim
>
> b) updatedb causes swapout
>
> c) prefer that openoffice/mozilla not get paged out when there's heavy
> pagecache demand.
>
> For a) we don't really have a solution. Some have been proposed but they
> could have serious downsides.
>
> For b) and c) we can tune the pageout-vs-cache reclaim tendency with
> /proc/sys/vm/swappiness, only nobody seems to know that.
>
> What else is there?

What I want is for purely sequential workloads which far exceed cache
size (dd, updatedb, tar czf /backup/home.nightly.tar.gz /home) to avoid
thrashing my entire desktop out of memory. I DON'T CARE if the tar
completed in 45 minutes rather than 80. (It wouldn't, anyways, because
it only needs about 5 MB of cache to get every bit of the speedup it was
going to get.) But the additional latency when I un-xlock in the
morning is annoying, and there is no benefit.

For a more useful example, ideally I *should not be able to tell* that
"dd if=/hde1 of=/hdf1" is running. [1] There is *no* benefit to cacheing
more than about 2 pages, under this workload. But with current kernels,
IME, that workload results in a gargantuan buffer cache and lots of
swapout of apps I was using 3 minutes ago. I've taken to walking away
for some coffee, coming back when it's done, and "sudo swapoff
/dev/hda3; sudo swapon -a" to avoid the latency that is so annoying when
trying to use bloaty apps.

[1] obviously I'll see some slowdown due to interrupts and PCI
bandwidth; that's not what I'm railing against, here.

-andy
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