Messages in this thread | | | Date | Tue, 17 Nov 1998 03:48:44 -0500 (EST) | From | Robert Minichino <> | Subject | RE: Elevator vs first-come-first-served |
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> "can actually decrease performance" > !! > can fail to help, sure, but other than the (wasted) cost of sorting, > how would it decrease performance? In general, that is, assuming some > probabilistic model of requests. I suppose you could construct a > Byzantine case.
When disk transaction frequency is at a high level in busy servers, the wasted cost of sorting does become an issue; also, it is not extremely rare for the OS to sort the data into the worst case for the controller's ordering algorithm, as the OS knows nothing (especially with SCSI-SCSI RAID controllers) about the true disk geometry. Being able to disable the sorting adds little to no complexity and and affords a small, but significant, increase in speed. Having more control over the sorting is even better. In benchmarks on Solaris with raw disk devices and a benchmarking program that simulated different ordering algorithms, throughput was highest to the disks (behind a cached controller with 64MB) when first-come first-served was used to service the randomly generated requests. I can rewrite and re-run the benchmark if anyone desires.
-- Robert Minichino Chief Engineer Denarius Enterprises, Inc.
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