Re: [PATCH] SLAB : use a multiply instead of a divide in obj_to_index()

Eric Dumazet
2006-12-04T22:45:14Z

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.