Messages in this thread | | | Date | Mon, 04 Dec 2006 23:45:28 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to_index() |
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David Miller a écrit : > From: Eric Dumazet <dada1@cosmosbay.com> > Date: Mon, 04 Dec 2006 22:34:29 +0100 > >> On a 200 MHz sparcv9 machine, the division takes 64 cycles instead of 1 cycle >> for a multiply. > > For UltraSPARC I and II (which is what this 200mhz guy probably is), > it's 4 cycle latency for a multiply (32-bit or 64-bit) and 68 cycles > for a 64-bit divide (32-bit divide is 37 cycles).
I must have Ultra-2 (running Solaris :( )
for (ui = 0 ; ui < 100000000 ; ui++) val += reciprocal_divide(ui, reciproc);
100000cb0: 83 31 20 00 srl %g4, 0, %g1 100000cb4: 82 48 40 12 mulx %g1, %l2, %g1 100000cb8: 83 30 70 20 srlx %g1, 0x20, %g1 100000cbc: 88 01 20 01 inc %g4 100000cc0: 80 a1 00 05 cmp %g4, %g5 100000cc4: 08 4f ff fb bleu %icc, 100000cb0 100000cc8: b0 06 00 01 add %i0, %g1, %i0
I confirm that this block uses 20 cycles/iteration, while next one uses 72 cycles/iteration
for (ui = 0 ; ui < 100000000 ; ui++) val += ui / value;
100000ca8: 83 31 20 00 srl %g4, 0, %g1 100000cac: 82 68 40 11 udivx %g1, %l1, %g1 100000cb0: 88 01 20 01 inc %g4 100000cb4: 80 a1 00 05 cmp %g4, %g5 100000cb8: 08 4f ff fc bleu %icc, 100000ca8 100000cbc: b0 06 00 01 add %i0, %g1, %i0
> > UltraSPARC-III and IV are worse, 6 cycles for multiply and 40/71 > cycles (32/64-bit) for integer divides. > > Niagara is even worse :-) 11 cycle integer multiply and a 72 cycle > integer divide (regardless of 32-bit or 64-bit). > > (more details in gcc/config/sparc/sparc.c:{ultrasparc,ultrasparc3,niagara}_cost). > > So this change has tons of merit for sparc64 chips at least :-) > > Also, the multiply can parallelize with other operations but it > seems that integer divide stalls the pipe for most of the duration > of the calculation. So this makes the divide even worse.
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