Date: Sun, 17 Sep 2006 19:33:06 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Eugene Kazarinov <kamuzon@milshop.ru> Cc: freebsd-stable@freebsd.org Subject: Re: Polling and em0 Message-ID: <20060917192528.P56201@fledge.watson.org> In-Reply-To: <519867a90609170920x51726e74k6f86f2661965fd03@mail.gmail.com> References: <op.tfzin5x2zq5pz4@sovaio.netoldies.com> <450CE15B.3060806@thebeastie.org> <519867a90609170920x51726e74k6f86f2661965fd03@mail.gmail.com>
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On Sun, 17 Sep 2006, Eugene Kazarinov wrote: >> Since 6.1 I have stopped using polling as a regular kernel seems to give >> good performance as compared to polling mode. In fact I used to get >> anything between 55-75megs/sec on 6.1 with regular cables but since >> upgraded to cat6 quality Ethernet cable I get practically full gigabit >> speed with no polling. Close to 97megs/sec is the performance I get out of >> my Dells with the em driver, and apparently the new em driver in 6-stable >> 6.2 is even faster. >> >> em(4) Dell to em(4) Dell >> dell1# cat /dev/zero | dd bs=1m | nc dell2 3000 >> ^C0+18456 records in >> 0+18455 records out >> 1209466880 bytes transferred in 12.459299 secs (97,073,429 bytes/sec) > > Do you meen that from 6.2 I dont need polling for fastest performance? There are two benefits to polling: - Moderating interrupt rates - Scheduling control over the amount of CPU dedicated to network tasks Modern gigabit cards have interrupt moderation, so in practice you get a polling-like effect with regard to interrupt rates. With significant performance improvements in the if_em driver over the last year, it could well be that the benefits of the scheduling control no longer provide much help. Gigabit network cards also require a very high polling rate in order to be effective, since the on-card buffers quickly fill -- as a result, unless the poll rate (driven by HZ) is set extremely high, polling may actually significant reduce performance by dramatically increasing packet loss under load. Robert N M Watson Computer Laboratory University of Cambridge
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