Messages in this thread | | | Date | Mon, 14 Aug 2000 16:38:01 +0200 | From | Jens Axboe <> | Subject | Re: Degrading disk read performance under 2.2.16 |
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On Mon, Aug 14 2000, Corin Hartland-Swann wrote: > I have tried this out, and found that the default settings were: > elevator ID=232 read_latency=128 write_latency=8192 max_bomb_segments=4 > > Can anyone point me to any documentation on this elevator code? I was > wondering what exactly the different variables do...
Heh, I don't think there is any documentation. But essentially the two latency variables set how many segments can supercede the request when the elevator sorts in new requests. The max_bomb_segments controls the max size of incoming writes, when there are read pendings for execution.
> I found that increasing the read_latency to match write_latency fixes the > problems perfectly - thanks for the pointer!
Ok good. Andrea, could we bump read latency in 2.2.17-pre? > > Increasing the read_latency and write_latency to 10,000,000 results in > similar throughput, but catastrophic seek performance:
Odd...
> Now, does anyone (Andrea in particular) know where the defaults are set? I
include/linux/blkdev.h, ELEVATOR_DEFAULTS.
> assume that setting read_latency to much lower than write_latency was an > accident, but can't find whereabouts these values are allocated in the > kernel. If this could be modified in the kernel then this would get disk > performance back up to scratch...
I can't imagine this being an accident, I'm sure Andrea had some data to backup the values chosen. But the current defaults do impose performance problems, as they did in 2.4, so I think that up'ing read latency is a good idea.
-- * Jens Axboe <axboe@suse.de> * SuSE Labs
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