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When some objects are allocated by one CPU but freed by another CPU we can consume lot of cycles doing divides in obj_to_index(). (Typical load on a dual processor machine where network interrupts are handled by one particular CPU (allocating skbufs), and the other CPU is running the application (consuming and freeing skbufs)) Here on one production server (dual-core AMD Opteron 285), I noticed this divide took 1.20 % of CPU_CLK_UNHALTED events in kernel. But Opteron are quite modern cpus and the divide is much more expensive on oldest architectures : On a 200 MHz sparcv9 machine, the division takes 64 cycles instead of 1 cycle for a multiply. Doing some math, we can use a reciprocal multiplication instead of a divide. If we want to compute V = (A / B) (A and B being u32 quantities) we can instead use : V = ((u64)A * RECIPROCAL(B)) >> 32 ; where RECIPROCAL(B) is precalculated to ((1LL << 32) + (B - 1)) / B Note : I wrote pure C code for clarity. gcc output for i386 is not optimal but acceptable : mull 0x14(%ebx) mov %edx,%eax // part of the >> 32 xor %edx,%edx // useless mov %eax,(%esp) // could be avoided mov %edx,0x4(%esp) // useless mov (%esp),%ebx [akpm@osdl.org: small cleanups] Signed-off-by: Eric Dumazet <dada1@cosmosbay.com> Cc: Christoph Lameter <clameter@sgi.com> Cc: David Miller <davem@davemloft.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
33 lines
846 B
C
33 lines
846 B
C
#ifndef _LINUX_RECIPROCAL_DIV_H
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#define _LINUX_RECIPROCAL_DIV_H
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#include <linux/types.h>
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/*
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* This file describes reciprocical division.
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*
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* This optimizes the (A/B) problem, when A and B are two u32
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* and B is a known value (but not known at compile time)
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*
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* The math principle used is :
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* Let RECIPROCAL_VALUE(B) be (((1LL << 32) + (B - 1))/ B)
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* Then A / B = (u32)(((u64)(A) * (R)) >> 32)
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*
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* This replaces a divide by a multiply (and a shift), and
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* is generally less expensive in CPU cycles.
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*/
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/*
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* Computes the reciprocal value (R) for the value B of the divisor.
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* Should not be called before each reciprocal_divide(),
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* or else the performance is slower than a normal divide.
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*/
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extern u32 reciprocal_value(u32 B);
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static inline u32 reciprocal_divide(u32 A, u32 R)
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{
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return (u32)(((u64)A * R) >> 32);
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}
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#endif
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