Messages in this thread | | | Date | Tue, 3 Dec 2002 07:21:49 +0100 | From | Willy Tarreau <> | Subject | Re: [PATCH] 2.4.20-rmap15a |
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On Mon, Dec 02, 2002 at 09:10:03PM -0200, Rik van Riel wrote: > On Mon, 2 Dec 2002, Willy Tarreau wrote: > > > - not one, but two elevators, one for read requests, one for write requests. > > - we would process one of the request queues (either reads or writes), and > > after a user-settable amount of requests processed, we would switch to the > > OK, lets for the sake of the argument imagine such an > elevator, with Read and Write queues for the following > block numbers: > > R: 1 3 4 5 6 20 21 22 100 110 111 > > W: 2 15 16 17 18 50 52 53 > > Now imagine what switching randomly between these queues > would do for disk seeks. Especially considering that some > of the writes can be "sandwiched" in-between the reads...
Well, I don't speak about "random switching". My goal is exactly to reduce seek time *and* to bound latency. Look at your example, considering that we put no limit on the number of consecutive requests, just processing them in order would give :
R(1), W(2), R(3,4,5,6), W(15,16,17,18), R(20,21,22), W(50,52,53), R(100,110,111)
This is about what is currently done with a single elevator. Now, if we try to detect long runs of consecutive accesses based on seek length, we could optimize it this way :
W(2), R(1-22), W(15-53), R(100-111) => we only do one backwards seek
And now, if you want to lower latency for a particular usage, with a 3:1 read/write ratio, this would give :
R(1,3,4), W(2), R(5,6,20), W(15), R(21,22,100), W(16), R(110,111), W(17-53)
Of course, this won't be globally optimal, but could perhaps help *some* processes to wait less time for their data, which is the goal of inserting read requests near the head of the queue, isn't it ?
BTW, just for my understanding, what would your example look like with the current elevator (choose the ordering you like) ?
Cheers, Willy
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