Achim Gsell wrote:
On Nov 23, 2010, at 12:15 AM, Simon Wilkinson wrote:
On 22 Nov 2010, at 23:06, Achim Gsell wrote:
3.) But if I first open 8 files and - after this is done - start writing
to these files sequentially, the problem occurs. The difference to 1.)
and 2.) is, that I have these 8 open files while the test is running.
This simulates the "putc-test" of bonnie++ more or less:
AFS is a write-on-close filesystem, so holding all of these files open
means that it is trying really hard not to flush any data back to the
fileserver. However, at some point the cache fills, and it has to start
writing data back. In 1.4, we make some really bad choices about which data
to write back, and so we end up thrashing the cache. With Marc Dionne's
work in 1.5, we at least have the ability to make better choices, but
nobody has really looked in detail at what happens when the cache fills, as
the best solution is to avoid it happening in the first place!
Sounds reasonable. But I have the same problem with a 9GB disk-cache, a 1GB
disk-cache, 1GB mem-cache and a 256kB mem-cache: I can write 6 GB pretty fast
then performance drops to< 3MB/s ...
We are always using memcache with only 64 or 256 MB, but I have seen this
problem, too. I think it's on the server side: Today's server have a lot of
memory and the data are written into the buffers first. Only when the buffers
rach the limit the operating system starts to really sync them out to the disks.
And with this huge amount of buffers you regularly see for some time the
performance going down. I suppose that during the sync the fileserver's writes
are hanging.
Hartmut
So long
Achim
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Institut fuer Plasmaphysik (IPP)
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