Messages in this thread | | | Date | Thu, 12 Feb 2004 21:27:14 +1100 | From | Nick Piggin <> | Subject | Re: ext2/3 performance regression in 2.6 vs 2.4 for small interl |
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Giuliano Pochini wrote:
>On 12-Feb-2004 Andrea Arcangeli wrote: > > >>the main difference is that 2.4 isn't in function of time, it's in >>function of requests, no matter how long it takes to write a request, >>so it's potentially optimizing slow devices when you don't care about >>latency (deadline can be tuned for each dev via >>/sys/block/*/queue/iosched/). >> > >IMHO it's the opposite. Transfer speed * seek time of some >slow devices is lower than fast devices. For example: > >Hard disk raw speed= 40MB/s seek time = 8ms >MO/ZIP raw speed= 3MB/s seek time = 25ms > >
I like accounting by time better because its accurate and fair for all types of devices, however I admit an auto tuning feature would be nice.
Say you allow 16 128K requests before seeking: The HD will run the requests for 50ms then seek (8ms). So this gives you about 86% efficiency. On your zip drive it takes 666ms, giving you 96%.
Now with AS, allowing 50ms of requests before a seek gives you the same for an HD, but only 66% for the MO drive. A CD-ROM will be much worse.
Auto tuning wouldn't be too hard. Just measure the time it takes for your seeking requests to complete and you can use the simple formula to allow users to specify a efficiency vs latency %age.
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