From owner-freebsd-arch  Mon Feb 18 19:20:13 2002
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Date: Mon, 18 Feb 2002 19:02:24 -0800 (PST)
From: Julian Elischer <julian@elischer.org>
To: Jeff Roberson <jroberson@chesapeake.net>
Cc: arch@freebsd.org
Subject: Re: Prioritized bio patches.
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this is kind of cool

Can you give any figures showing realtive effects on (say) buildworld
time when there is NICE work also on the system?
(I guess you must have doen it for a reason, so maybe you can give us some
examples of the result)

regards, julian


On Mon, 18 Feb 2002, Jeff Roberson wrote:

> I have developed an extension to the bio queue interface to support
> priorities. It may be useful in place of the nice value related sleep
> that is currently in bioqdisksort.  There are 6 queues in each bio queue
> head. The first is the default sorted queue.  It is used if the bio
> priority (bio_prio) is BIO_PRIO_DEFAULT.  Then there are 5 unsorted queues
> for use with different nice levels.
> 
> The priority is automatically picked at insert time based on the process
> nice value.  Un-nice processes still use the default IO queue to prevent
> starvation.  The lower priority IO is not sorted because it may be
> interrupted at any time by higher priority io, thus defeating the sort.
> This also saves a bit of structure bloat and processing time.
> 
> The main advantage over the current solution is that, if the system has
> spare IO cycles, nice processes can run at full speed.  Also, this was
> implemented in such a way that it is completely transparent to users of
> the bioq interfaces.  Since cam keeps a deep queue of IOs it may want to
> adjust it's priorities for the IO as well.  This would prevent a possible
> delay for high priority IO while cam works through a queue full of low
> priority IO. As it is though the response time should be reasonable.
> 
> This patch does not introduce any higher priority levels.  It would be
> easy to do, but I don't see any demand for it right now.  The patch is
> available at http://www.chesapeake.net/~jroberson/prio.patch
> 
> Thanks,
> Jeff
> 
> 
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