Santos, Jose Renato G (Jose Renato Santos)
2005-Apr-05 23:59 UTC
RE: [Xen-devel] MPI benchmark performance gap between native linux anddomU
>> -----Original Message----- >> From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk] >> Sent: Tuesday, April 05, 2005 8:47 AM >> To: Santos, Jose Renato G (Jose Renato Santos) >> Cc: Turner, Yoshio; Aravind Menon; >> Xen-devel@lists.xensource.com; 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 >> >>_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel