DragonFly BSD
DragonFly kernel List (threaded) for 2006-03
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Re: Disk Scheduler

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Sun, 19 Mar 2006 10:14:24 -0800 (PST)

:Has anyone looked at porting over the hybrid disk scheduler stuff that
:was done for freebsd ?
:I'm just now looking at it and I'm having a hard time merging it over
:after the latest buf changes that Matt did.
:Can anyone else take a look at this ?  I can post the diffs of what I
:currently have against DragonFly if that would help.

    This is a little late, but I have looked at the patch and it 
    certainly looks interesting.  BUT, there are two problems.

    First, I do not suggest that anyone attempt to port it due to
    the current byte-linearized buffer work that is going on.

    Second, unless I am mistaken I do not see any time-domain code
    to account for performance issues due to seeking in the HYBRID scheduler
    in the patch.   For example, if you have two users and each is supposed
    to get 50% of available disk bandwidth, and one is running a program
    which seeks the disk all over the place while the other is simply
    reading a single large linear file, I don't think HYBRID would actually
    apportion disk bandwidth properly between the two.  To do that properly
    would require detecting the situation where the linear reader is
    getting less then 50% of the expected (linear) bandwidth and then 
    forcing some sort of time-domain scheduling for that disk (kinda like
    time slices) to prevent the heavy-seeking from destroying performance 
    for the linear reader.

					Matthew Dillon 

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