Date: Wed, 19 Aug 1998 09:49:43 -0700 (PDT) From: Marc Slemko <marcs@znep.com> To: Wolfram Schneider <wosch@cs.tu-berlin.de> Cc: hackers@FreeBSD.ORG Subject: Re: sendfile() API? Message-ID: <Pine.GSO.4.02A.9808190942480.13979-100000@redfish> In-Reply-To: <19980819111809.A22887@caramba.cs.tu-berlin.de>
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On Wed, 19 Aug 1998, Wolfram Schneider wrote: > On 1998-08-17 18:17:34 +0000, Terry Lambert wrote: > > I have no doubt that it would be useful for static pages; I am just not > > convinced that pushing the data over a call boundary is the real overhead > > here. For an mmap'ed file, the only overhead is the copy from the VM > > buffer to the mbuf. The only way such a call could eliminated this > > overhead is by passing the VM buffers down as mbuf contents. This can > > be done (I did the code for it on the VMS NetWare server), but it's > > a lot of work. > > Who cares about data copy overhead? If the kernel spend 5% of the CPU > time in copying, there are still 95% free CPU cycles. > > Freefall.freebsd.org transfers 20KB/s with a CPU load less > than 0.01. Lots of people do and should care about it. freefall has such a tiny amount of web traffic that you could add an extra dozen copies to the path without it mattering. However, as you scale upwards by a few orders of magnitude, a bottleneck clearly appears on most current systems where you have excessive copying. Not all of this can be reflected in the CPU use of the process. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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