Date: Thu, 26 Jul 2012 22:52:08 -0700 From: Adrian Chadd <adrian@freebsd.org> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: current@freebsd.org Subject: Re: RFC: use EM_LEGACY_IRQ in if_lem.c ? Message-ID: <CAJ-VmokmjVnFKr-MXEB55p7qDPAro6AUVHuDL5UGifUZ-W8Yfw@mail.gmail.com> In-Reply-To: <20120725151403.GA33640@onelab2.iet.unipi.it> References: <20120724202019.GA22927@onelab2.iet.unipi.it> <CAJ-VmokG-%2BkjaOC2g2uvVX5z4eBtry_-L8nMFaOPBan9SSzyYQ@mail.gmail.com> <20120725151403.GA33640@onelab2.iet.unipi.it>
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On 25 July 2012 08:14, Luigi Rizzo <rizzo@iet.unipi.it> wrote: >> I suggest doing some digging to understand why. I bet we all know the >> answer, but it would be nice to have it documented and investigated. I >> bet em(4) isn't the only device that would benefit from this? > > I am not so sure i know the answer on bare iron (and my take is that the > difference is more or less irrelevant there), but in the virtualized case > the improvement is almost surely because the code used in FAST_INTR > has a couple of MMIO accesses to disable/enable interrupts on the > card while the taskqueue runs. These are expensive in a VM > (such accesses cost ~10K cycles each, even with hw support) Hm, really? Doing these register accesses to a virtualised em NIC in a VM is that expensive, or is there something else going on I don't understand? Adrian
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