Date: Sun, 21 Mar 2010 08:46:58 -0700 From: Julian Elischer <julian@elischer.org> To: Alexander Motin <mav@freebsd.org> Cc: FreeBSD-Current <freebsd-current@freebsd.org>, freebsd-arch@freebsd.org Subject: Re: Increasing MAXPHYS Message-ID: <4BA63F72.5000806@elischer.org> In-Reply-To: <4BA62757.7090400@FreeBSD.org> References: <4BA4E7A9.3070502@FreeBSD.org> <201003201753.o2KHrH5x003946@apollo.backplane.com> <891E2580-8DE3-4B82-81C4-F2C07735A854@samsco.org> <4BA52179.9030903@FreeBSD.org> <4BA532FF.6040407@elischer.org> <4BA62757.7090400@FreeBSD.org>
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Alexander Motin wrote: > Julian Elischer wrote: >> In the Fusion-io driver we find that the limiting factor is not the >> size of MAXPHYS, but the fact that we can not push more than >> 170k tps through geom. (in my test machine. I've seen more on some >> beefier machines), but that is only a limit on small transacrtions, >> or in the case of large transfers the DMA engine tops out before a >> bigger MAXPHYS would make any difference. > > Yes, GEOM is quite CPU-hungry on high request rates due to number of > context switches. But impact probably may be reduced from two sides: by > reducing overhead per request, or by reducing number of requests. Both > ways may give benefits. > > If common opinion is not to touch defaults now - OK, agreed. (Note, > Scott, I have agreed :)) But returning to the original question, does > somebody knows real situation when increased MAXPHYS still causes > problems? At least to make it safe. well I know we havn't tested our bsd driver yet with MAXPHYS > 128KB at this time.. Must try that some time :-)
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