Re: [Bench] New benchmark showing fileserver problem in 2.4.12

[Andreas Dilger <adilger@turbolabs.com>]

On Oct 17, 2001  23:06 +1000, Robert Cohen wrote:
> Factor 1: the performance problems only occur when you are rewriting an
> existing file in place. That is writing to an existing file which is
> opened without O_TRUNC. Equivalently, if you have written a file and
> then seek'ed back to the beginning and started writing again.
> 
> Evidence: in the report I posted yesterday, the test I was using
> involved 5 clients rewriting 30 Meg files on a 128 Meg machine. The
> symptom  was that after about 10 seconds, the throughput as shown by
> vmstat "bo" drops sharply and we start getting reads occuring as shown
> by the "bi" figure.

Just a guess - if you are getting reads that are about the same as writes,
then it would indicate that the code is doing "read-modify-write" for the
existing file data rather than just "write".  This would be caused by not
writing only full-sized aligned blocks to the files.

As to why this is happening only over the network - it may be that you are
are unable to send an even multiple of the blocksize over the network (MTU)
and this is causing fragmented writes.  Try using a smaller block size like
4k or so to see if it makes a difference.

Another possibility is that with 8k chunks you are needing order-1
allocations to receive the data and this is causing a lot of searching for
buffers to free.

Cheers, Andreas
--
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert