Date: Wed, 5 May 2004 17:23:30 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: Bruce Evans <bde@zeta.org.au> Cc: Gerrit Nagelhout <gnagelhout@sandvine.com> Subject: RE: 4.7 vs 5.2.1 SMP/UP bridging performance Message-ID: <16537.23378.375946.857908@grasshopper.cs.duke.edu> In-Reply-To: <20040505222636.H15444@gamplex.bde.org> References: <FE045D4D9F7AED4CBFF1B3B813C85337021AB377@mail.sandvine.com> <20040505222636.H15444@gamplex.bde.org>
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Bruce Evans writes: > > Athlon XP2600 UP system: !SMP case: 22 cycles SMP case: 37 cycles > Celeron 366 SMP system: 35 48 > > The extra cycles for the SMP case are just the extra cost of a one lock > instruction. Note that SMP should cost twice as much extra, but the > non-SMP atomic_store_rel_int(&slock, 0) is pessimized by using xchgl > which always locks the bus. After fixing this: > > Athlon XP2600 UP system: !SMP case: 6 cycles SMP case: 37 cycles > Celeron 366 SMP system: 10 48 > > Mutexes take longer than simple locks, but not much longer unless the > lock is contested. In particular, they don't lock the bus any more > and the extra cycles for locking dominate (even in the !SMP case due > to the pessimization). > > So there seems to be something wrong with your benchmark. Locking the > bus for the SMP case always costs about 20+ cycles, but this hasn't > changed since RELENG_4 and mutexes can't be made much faster in the > uncontested case since their overhead is dominated by the bus lock > time. > Actually, I think his tests are accurate and bus locked instructions take an eternity on P4. See http://www.uwsg.iu.edu/hypermail/linux/kernel/0109.3/0687.html For example, with your test above, I see 212 cycles for the UP case on a 2.53GHz P4. Replacing the atomic_store_rel_int(&slock, 0) with a simple slock = 0; reduces that count to 18 cycles. If its really safe to remove the xchg* from non-SMP atomic_store_rel*, then I think you should do it. Of course, that still leaves mutexes as very expensive on SMP (253 cycles on the 2.53GHz from above). Drew
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