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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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