Date: Wed, 22 Mar 2006 19:23:00 -0700 From: Scott Long <scottl@samsco.org> To: Luigi Rizzo <rizzo@icir.org> Cc: Poul-Henning Kamp <phk@phk.freebsd.dk>, current@freebsd.org Subject: Re: interesting(?) data on network interrupt servicing Message-ID: <44220684.80300@samsco.org> In-Reply-To: <20060322132739.C42341@xorpc.icir.org> References: <20060322131448.A42341@xorpc.icir.org> <11680.1143062324@critter.freebsd.dk> <20060322132739.C42341@xorpc.icir.org>
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Luigi Rizzo wrote: > On Wed, Mar 22, 2006 at 10:18:44PM +0100, Poul-Henning Kamp wrote: > >>In message <20060322131448.A42341@xorpc.icir.org>, Luigi Rizzo writes: >> >> >>>>> if (thread) >>>>> isrc->is_pic->pic_disable_source(isrc, PIC_EOI); >>>>> >>>>>I have no idea, though, why the other pic_disable_source() >>>>>is so expensive. The 4-5k TSC ticks are approx 3us >>>>> >>>>>Any clues ? >>>> >>>>ISA bus access. >>> >>>yeah but this is a modern laptop with an apic, >>>not an 8259... i don't think it has any ISA bus unless there is >>>some strange emulation going on ? >> >>There is, how else would it boot MS-DOS ? > > > so anything we can do in the kernel config to remove that ? > (i don't have here the config Paolo is using...) > I've done extensive TSC measurements in here too about 18 months ago (see the commit logs for an optimization that I put in). Part of the expense is the indirect function call, and part of it is the memory read. I'm not sure if I completely agree with John that the read is over PCI, I think that for most cases it'll be on the front side bus. But in any case, it's an uncached read, so it's expensive. I like John's patch for getting rid of it. Beyond that, the other expenses come from using the icu_lock spinlock, and John and I think that there might be ways to reduce that. INTR_FAST handlers bypass a lot of this too. The only thing left after all of that is the bintime calls and the indirect function calls to the PIC/APIC code. Those are there as part of the PIC abstraction, and there is really no way around them. I'd really like to see FreeBSD be able to get from int assertion to driver interrupt handler in 1000 ticks or less (at least for INTR_FAST), so any suggestions are welcome. Scott
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