Re: Drop caches - is this safe behavior?

[mike <mike503@gmail.com>]

thanks for the reply... few comments

On 8/23/07, Chris Snook <csnook@redhat.com> wrote:

> This widespread fallacy has been false for a long time.  The kernel will
> swap instead of dropping caches if it believes that doing so will
> improve performance.  It uses heuristics for this, and sometimes guesses
> wrong, but it's not a bad thing.  Consider the memory used in the
> initialization and shutdown routines for an application.  In normal
> operation, you're never using it, so it's much better to swap this out
> than to drop caches of a file you've actually accessed recently.  It is
> completely normal to use have a small amount of swap utilization for
> precisely this reason.  All you're doing by dropping caches is hurting
> performance, probably by a lot.  The drop_caches patch was resisted for
> a very long time because we knew people would use it to shoot themselves
> in the foot.  It should only be used for debugging or benchmarking.

with this going right now, i have 0 megs of swap being used, tons of
physical memory available. basically what i would expect to see. i
have the PHP fastcgi engines killing themselves after only 50 requests
(aggressive) - its their effort to keep memory leaks away. i have
tried both aggressive and non-aggressive request numbers.

> "Rampant and greedy" is correct behavior, as long as it doesn't harm
> performance.  Usually it helps performance, but if it does harm
> performance, let us know, since that would be a bug we need to fix.  By
> dropping caches of NFS-backed files on multiple systems, you're moving
> the load from several underutilized systems to one heavily-utilized system.

it seems to affect performance. that's one of the reasons i am trying this out.

> Safety aside, it's harmful no matter where you do it.  Forget about
> drop_caches.  Don't use it.  It wasn't meant for your use case.
>
> If you think the system is doing the wrong thing (and it doesn't sound
> like it is) you should be tweaking the vm.swappiness sysctl.  The
> default is 60, but lower values will make it behave more like you think
> it should be behaving, though you'll still probably see a tiny bit of
> swap usage.  Of course, if your webservers are primarily serving up
> static content, you'll want a higher value, since swapping anonymous
> memory will leave more free for the pagecache you're primarily working with.

i have played with swappiness, i had it down to 10 at one point.

i have the drop_caches going every 5 minutes. the servers seem to be
running well. i had complaints after maybe 24 hours that things began
to lag on the web side. this has been running great so far.

i am also trying out a drop_caches with swappiness of 0 too on one
machine to see how that works.

it doesn't seem to be hurting anything currently. the NFS server is
performing with plenty of idle CPU, webservers have free memory, not
stuck up in swap that seems to not necessarily recycle nicely.

i'm trying deadline scheduler, the SLUB allocator, all sorts of things
here. sysctl configuration for tcp tweaks - with and without. nfs v3
and nfs v4. this actually seems to work though...

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>