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Eric Dumazet 6a2d7a955d [PATCH] SLAB: use a multiply instead of a divide in obj_to_index() ... 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>		18 years ago
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acpi	[PATCH] slab: remove kmem_cache_t	18 years ago
asm-alpha	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-arm	Merge branch 'i2c-for-linus' of git://jdelvare.pck.nerim.net/jdelvare-2.6	18 years ago
asm-arm26	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-avr32	Merge branch 'for-linus' of git://www.atmel.no/~hskinnemoen/linux/kernel/avr32	18 years ago
asm-cris	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-frv	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-generic	Make sure we populate the initroot filesystem late enough	18 years ago
asm-h8300	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-i386	Merge ../linus	18 years ago
asm-ia64	[IA64] enable trap code on slot 1	18 years ago
asm-m32r	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-m68k	[PATCH] Sun3: General updates	18 years ago
asm-m68knommu	[PATCH] cleanup asm/setup.h userspace visibility	18 years ago
asm-mips	[MIPS] IP27: Don't drag <asm/sn/arch.h> into topology.h.	18 years ago
asm-parisc	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-powerpc	Merge git://git.kernel.org/pub/scm/linux/kernel/git/paulus/powerpc	18 years ago
asm-ppc	Merge branch 'for_paulus' of master.kernel.org:/pub/scm/linux/kernel/git/galak/powerpc	18 years ago
asm-s390	Merge branch 'for-linus' of git://git390.osdl.marist.edu/pub/scm/linux-2.6	18 years ago
asm-sh	sh: SH-MobileR SH7722 CPU support.	18 years ago
asm-sh64	[PATCH] cleanup asm/setup.h userspace visibility	18 years ago
asm-sparc	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-sparc64	[SPARC64]: Fix several kprobes bugs.	18 years ago
asm-um	[PATCH] UML: add generic BUG support	18 years ago
asm-v850	[PATCH] tty: preparatory structures for termios revamp	18 years ago
asm-x86_64	Merge ../linus	18 years ago
asm-xtensa	[PATCH] xtensa: fix system call interface	18 years ago
crypto	[CRYPTO] lib: table driven multiplications in GF(2^128)	18 years ago
keys	…
linux	[PATCH] SLAB: use a multiply instead of a divide in obj_to_index()	18 years ago
math-emu	…
media	V4L/DVB (4935): Added the capability of selecting fm gain by tuner	18 years ago
mtd	Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6	19 years ago
net	[HAMRADIO]: Fix baycom_epp.c compile failure.	18 years ago
pcmcia	[PATCH] pcmcia: multifunction card handling fixes	18 years ago
rdma	IB: Add DMA mapping functions to allow device drivers to interpose	18 years ago
rxrpc	…
scsi	[PATCH] slab: remove kmem_cache_t	18 years ago
sound	Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6	18 years ago
video	[PATCH] video: pm3fb macros fix	18 years ago
Kbuild	…