Date: Wed, 26 Jul 2006 20:27:01 +0200 From: "Attilio Rao" <attilio@freebsd.org> To: "John Baldwin" <jhb@freebsd.org> Cc: freebsd-arch@freebsd.org Subject: Re: [PATCH] Mantaining turnstile aligned to 128 bytes in i386 CPUs Message-ID: <3bbf2fe10607261127p3f01a6c3w80027754f7d4e594@mail.gmail.com> In-Reply-To: <3bbf2fe10607251004wf94e238xb5ea7a31c973817f@mail.gmail.com> References: <3bbf2fe10607250813w8ff9e34pc505bf290e71758@mail.gmail.com> <3bbf2fe10607250814m1a476f09p2d962dedc0c99be1@mail.gmail.com> <200607251232.51230.jhb@freebsd.org> <3bbf2fe10607251004wf94e238xb5ea7a31c973817f@mail.gmail.com>
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2006/7/25, Attilio Rao <attilio@freebsd.org>: > 2006/7/25, John Baldwin <jhb@freebsd.org>: > > On Tuesday 25 July 2006 11:14, Attilio Rao wrote: > > > 2006/7/25, Attilio Rao <attilio@freebsd.org>: > > > > Hi, > > > > Intel documentation points out that having a 128-bytes aligned > > > > syncronizing primitive (which fits in a cache line) will minimize the > > > > traffic for cache bus, so this patch implements an alignment for i386 > > > > on turnstiles. > > > > > > > > Any comments, feedbacks? > > > > > > Oh, sorry, I've unforgotten the diff. > > > > > > Attilio > > > > I think a better approach would be to stick turnstiles (and sleepqueues) in a > > UMA zone and specify cache-size alignment to the zone. However, turnstiles > > aren't really sychronization primitives in that you don't spin on a variable > > inside the structure, and I think it's the spinning and avoiding bouncing > > cache lines around that Intel's documentation is really about. In that case, > > the things you want aligned are things like mutexes, rwlocks, etc. > > Well, I think that this is referred in particular to the latter issue > you mentioned. > Spinning is not really concerned to cache bus issues (more, in > particular, datapath latency). > With this point of view, turnstiles (as sleepqueues) are passed around > CPUs more than a mutex/rwlock (or a cv), I guess, so I was thinking > that it's better optimizing turnstile than the real syncronizing > primitive itself. This is a patch which let turnstiles/sleepqueues using an UMA zone. I've tried in my 6.1R branch and it works quite fine, so this HEAD version might be alright (I've not tried yet, so please test): http://users.gufi.org/~rookie/works/patches/uma_sync.diff It, obviously, set default alignment for i386 at 128 bytes. Any comments, feedbacks, ideas, are welcome. Attilio PS: I know that I could simplify *_alloc(), *_free() routines implementing init/fini but it is simpler and more optimized having things like so. -- Peace can only be achieved by understanding - A. Einstein
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