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

["Santos, Jose Renato G (Jose Renato Santos)" <joserenato.santos@xxxxxx>]

>> -----Original Message-----
>> From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx] 
>> Sent: Tuesday, April 05, 2005 8:47 AM
>> To: Santos, Jose Renato G (Jose Renato Santos)
>> Cc: Turner, Yoshio; Aravind Menon; 
>> Xen-devel@xxxxxxxxxxxxxxxxxxx; xuehai zhang; G John Janakiraman
>> Subject: 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.
>> 

  The question is if this 'suitable default' may be not suitable
anymore, because of the true_size lying trick.
  Probably this is non issue in most instalations but maybe a latent
error in network intensive applications. Just keep this in the back of
your mind in case a lack of memory problem for network applications
arises in the future.

>> 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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