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Re: [Xen-devel] MPI benchmark performance gap between native linux anddomU




On 5 Apr 2005, at 16:23, Santos, Jose Renato G (Jose Renato Santos) wrote:

In which version the 'truesize' field was changed to report less than
a page?
  We were using 2.0.3 when we found this problem.
  I agree this trick will prevent the early overflow of the receive
buffer.
However, I am thinking if there is no other side effect of lying about
the true size of the buffer to the kernel.
  Would bad things happen if the kernel believes that is using less
memory than it is really using.
For example, would it be possible for the kernel to exhaust memory for
network intensive application with a large number of open connections ?

I guess it would be easier to provoke trouble, but in any case the default advertised window and socket buffer allocation are not affected dynamically by system-wide memory pressure. Per-sockbuf limits are set to a 'suitable default' at boot-time according to amount of RAM detected, but after that they have to be manually reset by the user.

So I don't think we are breaking any carefully-tuned dynamically-balanced memory allocation algorithms here. :-)

By setting the true size (4kB) we are far more likely to throw network performance off, as the TCP stack will not have been tuned with such large packet overheads in mind.

 -- Keir


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