Date: Sun, 21 Mar 2010 20:54:59 -0400 From: jhell <jhell@DataIX.net> To: Alexander Motin <mav@freebsd.org> Cc: FreeBSD-Current <freebsd-current@freebsd.org>, Julian Elischer <julian@elischer.org>, freebsd-arch@freebsd.org Subject: Re: Increasing MAXPHYS Message-ID: <alpine.BSF.2.00.1003212045540.16103@qvfongpu.qngnvk.ybpny> 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>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sun, 21 Mar 2010 10:04, mav@ 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. > > I played with it on one re-compile of a kernel and for the sake of it DFLTPHYS=128 MAXPHYS=256 and found out that I could not cause a crash dump to be performed upon request (reboot -d) due to the boundary being hit for DMA which is 65536. Obviously this would have to be adjusted in ata-dma.c. I suppose that there would have to be a better way to get the real allowable boundary from the running system instead of setting it statically. Other then the above I do not see a reason why not... It is HEAD and this is the type of experimental stuff it was meant for. Regards, -- jhell
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?alpine.BSF.2.00.1003212045540.16103>