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SubjectRe: elevator algorithm bug in ll_rw_blk.c

On Sun, 15 Nov 1998, Martin Mares wrote:

> Hello,
>
> > But that's a *one-way* elevator. Ideal elevators are two-way, aren't they?
>
> Using two-way elevators for disk block sorting is probably highly suboptimal
> since most disks are painfully slow when reading blocks backwards.

Hi Martin, how are you going ?

I just select your posting to add my 0.2 euro to this thread. No great
disagreement with you this time. ;-)

My understanding of Linux architecture is that IO blocks coalescing only
occurs in ll_rw_blk.c. If it fails the coalescing then Linux will just do
1 IO for each block. When you have lots of memory, it may happen the
number of entries in the request array (NR_REQUEST) be not enough for
this coalescing to occur in some situations that may not be rare given the
way the buffer cache actually work.

In my opinion, it is a lot more important in Linux that the coalescing
does occur than the ordering of requests be really the best possible one.
Obviously the ordering is also important, even when using modern SCSI
disks that have a very well optimized seek algorithm.

Regards,
Gerard.


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