Date: Wed, 24 Oct 2012 09:57:18 -1000 (HST) From: Jeff Roberson <jroberson@jroberson.net> To: Attilio Rao <attilio@freebsd.org> Cc: Adrian Chadd <adrian@freebsd.org>, src-committers@freebsd.org, Andre Oppermann <andre@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org, Jim Harris <jim.harris@gmail.com> Subject: Re: svn commit: r242014 - head/sys/kern Message-ID: <alpine.BSF.2.00.1210240956210.1947@desktop> In-Reply-To: <CAJ-FndAJwuD=oUHyVK4xHoZNrf0r%2Bq2WxZ9rUMW%2B-zzMo_8QuA@mail.gmail.com> References: <201210241836.q9OIafqo073002@svn.freebsd.org> <CAJ-VmonpdJ445hXVaoHqFgS0v7QRwqHWodQrVHm2CN9T661www@mail.gmail.com> <CAJP=Hc9XmvfW3MrDjvK15OAx1fyfjPk%2BEhqHUOzoEpChu5imtg@mail.gmail.com> <50883EA8.1010308@freebsd.org> <CAJ-FndAJwuD=oUHyVK4xHoZNrf0r%2Bq2WxZ9rUMW%2B-zzMo_8QuA@mail.gmail.com>
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On Wed, 24 Oct 2012, Attilio Rao wrote: > On Wed, Oct 24, 2012 at 8:16 PM, Andre Oppermann <andre@freebsd.org> 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 > > This is wrong because it doesn't give padding. > >> 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 > > What is this macro supposed to do? I don't understand that from your > description. > >> 3. embed __aligned(CACHE_LINE_SIZE) into struct mtx itself so it >> automatically gets aligned in all cases, even when dynamically >> allocated. > > This works but I think it is overkill for structures including sleep > mutexes which are the vast majority. So I wouldn't certainly be in > favor of such a patch. I agree. For locks with little contention we probably want smaller structures. For example, you wouldn't want to put a huge lock in every file descriptor. It would be nice to have an automatic way to pad every global lock though. I think it should be done as needed. Jeff > > Attilio > > > -- > Peace can only be achieved by understanding - A. Einstein >
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