Re: ~500 megs cached yet 2.6.5 goes into swap hell

[Andrew Morton]

Andy Isaacson <adi@xxxxxxxxxxxxx> wrote:
>
> 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.

What kernel version are you using?  If 2.6, what value of
/proc/sys/vm/swappiness?

> For a more useful example, ideally I *should not be able to tell* that
> "dd if=/hde1 of=/hdf1" is running.

I just did a 4GB `dd if=/dev/sda of=/x bs=1M' on a 1GB 2.6.6-rc2-mm2
swappiness=85 machine here and there was no swapout at all.

Probably your machine has less memory.  But without real, hard details
nothing can be done.

> There is *no* benefit to cacheing
> more than about 2 pages, under this workload.

Sure, we could do better things with the large streaming files, although
the risk of accidentally screwing up particular workloads is high.

But the use-once logic which we have in there at present does handle these
cases quite well.

>  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.

What kernel, what system specs, what swappiness setting?
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