Priorities for I/O

[Kurt Garloff <garloff@suse.de>]

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Hi Jens,

one of the shortcomings of Linux process priority system is that it only
affects CPU resources (and even those are not affected strongly enough for
some applications). 
As soon as processes waits for I/O, they are all equal.

I wonder how difficult it was to just add priorities to your I/O scheduler.
Basically, I think everything is there: You sort requests already and we
have deadlines. For sorting scores are used.
So the idea is: Why not just give I/O submitted on behalf of -19 processes
(and RT) some higher score and a shorter deadline than +19 ones?
Probably, it would only affect reads, as there we have the processes wait
on them; writes are often triggered asynchronously anyway.

This should have the effects that we want: When there's no fight for I/O
bandwidth, everybody just gets maximum performance as the I/O scheduler's
queue will be short. As soon as processes fight for reads, unniced processes
have a higher chance of getting served first.

Just think of nightly updatedb on a webserver for a real-world example why
this may matter.

Looks like something not too difficult to do, but my current knowledge on
2.5 code is somewhat sparse :-(

Regards,
-- 
Kurt Garloff  <garloff@suse.de>                          Eindhoven, NL
GPG key: See mail header, key servers                        SuSE Labs
SuSE Linux AG, Nuernberg, DE                            SCSI, Security

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