Date: Wed, 24 Oct 2012 22:42:54 +0200 From: Andre Oppermann <andre@freebsd.org> To: Alexander Motin <mav@FreeBSD.org> Cc: svn-src-head@freebsd.org, Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, Jim Harris <jim.harris@gmail.com>, svn-src-all@freebsd.org Subject: Re: svn commit: r242014 - head/sys/kern Message-ID: <508852CE.5030807@freebsd.org> In-Reply-To: <508841DC.7040701@FreeBSD.org> References: <201210241836.q9OIafqo073002@svn.freebsd.org> <CAJ-VmonpdJ445hXVaoHqFgS0v7QRwqHWodQrVHm2CN9T661www@mail.gmail.com> <CAJP=Hc9XmvfW3MrDjvK15OAx1fyfjPk%2BEhqHUOzoEpChu5imtg@mail.gmail.com> <50883EA8.1010308@freebsd.org> <508841DC.7040701@FreeBSD.org>
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On 24.10.2012 21:30, Alexander Motin wrote: > On 24.10.2012 22:16, Andre Oppermann wrote: >> On 24.10.2012 20:56, Jim Harris wrote: >>> On Wed, Oct 24, 2012 at 11:41 AM, Adrian Chadd <adrian@freebsd.org> >>> wrote: >>>> On 24 October 2012 11:36, Jim Harris <jimharris@freebsd.org> wrote: >>>> >>>>> Pad tdq_lock to avoid false sharing with tdq_load and tdq_cpu_idle. >>>> >>>> Ok, but.. >>>> >>>> >>>>> struct mtx tdq_lock; /* run queue lock. */ >>>>> + char pad[64 - sizeof(struct mtx)]; >>>> >>>> .. don't we have an existing compile time macro for the cache line >>>> size, which can be used here? >>> >>> Yes, but I didn't use it for a couple of reasons: >>> >>> 1) struct tdq itself is currently using __aligned(64), so I wanted to >>> keep it consistent. >>> 2) CACHE_LINE_SIZE is currently defined as 128 on x86, due to >>> NetBurst-based processors having 128-byte cache sectors a while back. >>> I had planned to start a separate thread on arch@ about this today on >>> whether this was still appropriate. >> >> See also the discussion on svn-src-all regarding global struct mtx >> alignment. >> >> Thank you for proving my point. ;) >> >> Let's go back and see how we can do this the sanest way. These are >> the options I see at the moment: >> >> 1. sprinkle __aligned(CACHE_LINE_SIZE) all over the place >> 2. use a macro like MTX_ALIGN that can be SMP/UP aware and in >> the future possibly change to a different compiler dependent >> align attribute >> 3. embed __aligned(CACHE_LINE_SIZE) into struct mtx itself so it >> automatically gets aligned in all cases, even when dynamically >> allocated. >> >> Personally I'm undecided between #2 and #3. #1 is ugly. In favor >> of #3 is that there possibly isn't any case where you'd actually >> want the mutex to share a cache line with anything else, even a data >> structure. > > I'm sorry, could you hint me with some theory? I think I can agree that cache line sharing can be a > problem in case of spin locks -- waiting thread will constantly try to access page modified by other > CPU, that I guess will cause cache line writes to the RAM. But why is it so bad to share lock with > respective data in case of non-spin locks? Won't benefits from free regular prefetch of the right > data while grabbing lock compensate penalties from relatively rare collisions? Cliff Click describes it in detail: http://www.azulsystems.com/blog/cliff/2009-04-14-odds-ends For a classic mutex it likely doesn't make much difference since the cache line is exclusive anyway while the lock is held. On LL/SC systems there may be cache line dirtying on a failed locking attempt. For spin mutexes it hurts badly as you noted. Especially on RW mutexes it hurts because a read lock dirties the cache line for all other CPU's. Here the RW mutex should be on its own cache line in all cases. -- Andre
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