On Thu, Jul 28, 2005 at 11:08:12AM +0100, Greig A Cowan wrote:
> Hi Kostas,
>
> Thanks for getting back to me.
>
> > Are the java processes really use that much memory or is it the buffer
> > cache? The kernel will try to cache as many files as possible so high
> > memory usage is normal (your ganglia page isn't visible from the outside
> > world)
>
> Sorry about the ganglia page, I thought that was now visible. I think you
> are correct though, we are seeing high usage of cached memory. Do you
> think it would be useful to tune our virtual memory settings as you
> mentioned in your previous email (RE: Kernels)?
Have a look at this paper: http://people.redhat.com/nhorman/papers/rhel3_vm.pdf
It's RHEL3 specific but it will give you an idea about what you need to tune.
Is it really a problem that the file cache is taking all the unused memory?
As long as the pages are "clean" the kernel can throw them out easilly without
any problems. You might want to tune vm.bdflush though to be more aggresive so
dirty pages are written to disk as fast as possible.
> During the FTS tests we were running with 8 files and 0 (?) streams. Can
> anyone point me to a webpage with information regarding the difference
> between streams and files? With this setup we were seeing ~200Mb/s into
> our site.
With n multiple streams dcache writes data like:
stream: write 10K, seek ahead (n-1)*10K, write 10K, ....
.......
With 1 stream you get:
stream: write 10K, write 10K, ...
If the data from all the streams doesn't arrive at the same time
you end up with writes all over the place and your disk IO suffers
as a result. The 3ware raid cards that we use at IC get around
~70MB/sec at sequencial IO and around ~1-5MB/sec at random IO in
RAID5. It's only a guess in my part that the parallel streams cause
that much random IO or not since i didn't had that much time to test
different settings during the FTS transfers :(
Cheers,
Kostas
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