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KernelTrap: Where The Anticipatory Scheduler Shines

Feb 25, 2003, 19:00 (0 Talkback[s])

[ Thanks to Jeremy Andrews for this link. ]

"Andrew Morton recently posted some interesting benchmarks comparing the current 2.4 IO scheduler, a 'hacked' version of the deadline IO scheduler in 2.5.61, the CFQ scheduler, and the anticipatory scheduler. Offering a succinct 'executive summary' of his results, Andrew said, 'the anticipatory scheduler is wiping the others off the map, and 2.4 is a disaster.' Indeed, in many of the tests the other IO schedulers were measured in minutes, whereas the anticipatory IO scheduler was measured in mere seconds.

"Andrea Arcangeli responded to these tests pointing out that they do not in any way highlight the benefits of the CFQ scheduler, which instead is designed to maintain minimal worst case latency on each and every IO read and write. Andrea explains, 'CFQ is made for multimedia desktop usage only, you want to be sure mplayer or xmms will never skip frames, not for parallel cp reading floods of data at max speed like a database with zillon of threads.' This lead to an interesting discussion in which Andrew suggested that such programs employ a broken design which should be fixed directly, rather than working around them in the IO scheduler.

"Andrew's latest -mm release, 2.5.62-mm3, still includes all three of the 2.5 kernel's IO schedulers. The default is 'as', but you can also select 'cfq' or 'deadline' from the kernel boot commandline..."

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