Date: Sun, 24 Dec 2006 22:08:45 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Robert Watson <rwatson@FreeBSD.org> Cc: cvs-src@FreeBSD.org, Scott Long <scottl@samsco.org>, src-committers@FreeBSD.org, cvs-all@FreeBSD.org, John Polstra <jdp@polstra.com> Subject: Re: cvs commit: src/sys/dev/bge if_bge.c Message-ID: <20061224211712.W25632@delplex.bde.org> In-Reply-To: <20061224085231.Y37996@fledge.watson.org> References: <XFMail.20061223102713.jdp@polstra.com> <20061223213014.U35809@fledge.watson.org> <458E11AE.2000004@samsco.org> <20061224085231.Y37996@fledge.watson.org>
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On Sun, 24 Dec 2006, Robert Watson wrote: >> From the perspective of optimizing these particular paths, small packet >> sizes > best reveal processing overhead up to about the TCP/socket buffer layer on > modern hardware (DMA, etc). The uni/bidirectional axis is interesting > because it helps reveal the impact of the direct dispatch vs. netisr dispatch > choice for the IP layer with respect to exercising parallelism. I didn't > explicitly measure CPU, but as the configurations max out the CPUs in my test > bed, typically any significant CPU reduction is measurable in an improvement > in throughput. For example, I was easily able to measure the CPU reduction > in switching from using the socket reference to the file descriptor reference > in sosend() on small packet transmit, which was a relatively minor functional > change in locking and reference counting. Be careful with micro-optimizations. I saw a single change (adding about 1K in unrelated code that is never executed) give a pessimization of 15% for tx bge (from 360 kpps to 300 kpps). Before that I was trying harder than now to find optimizations involving avoiding copying, and thought that I had increased the speed from 330 kpps to 360 kpps by removing things, but I may have just increased the speed by moving cache phenomena. The phenomena in this case seem to be related to instructions more than data and I suspect that they are very MD. The machine that has them doesn't support APIC or ACPI, so hwpmc cannot do anything useful on it. Brucehome | help
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