The CFQ "low latency" mode
Posted Oct 8, 2009 23:20 UTC (Thu) by giraffedata
Parent article: The CFQ "low latency" mode
Normally the scheduler will try to delay many new I/O requests for a short time in the hope that they can be joined with other requests which may come shortly thereafter. This behavior will minimize disk seeks and maximize I/O request size, so it is clearly good for throughput.
No, that's not clear at all. Minimizing disk seeks and maximizing I/O request size is clearly good for disk efficiency -- minimizing disk utilization for a given workload -- but for throughput to be meaningful, utilization has to be about 100%. When that's the case, I/O backs up into the Linux I/O queue so that no extra delays are necessary in order to join requests with other requests.
You just can't improve throughput by deliberately letting the disk sit idle.
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