Andi Kleen wrote:
>
> "Jeff V. Merkey" <jmerkey@timpanogas.com> writes:
>
> > If I increase the size of the LRU in NWFS to a size above 3000 buffers,
> > Linux starts getting SLOWER .... SLOOOWWWWWWERRRR......
> > SLOOOOOOOWWWWWWEEEERR ..... which is counter intuitive since more cache
> > should get faster performance. The performance degredation as I
> > increase the LRU in size is @ log2N, which would seem to indicate that
> > somewhere down inside of linux, as some linked lists start getting
> > longer, Linux really bogs down and exhibits asymtopic behavior.
>
> The culprit is probably the elevator code in ll_rw_blk, which uses
> a single linked list, and possible the related queue walking code in
> the SCSI driver. These queues are walked all the time to coalesce read/write
> requests. You can check by booting with profile=2
> and then using /usr/sbin/readprofile to get profile logs. The bad
> routine should clearly show off.
Andi,
I'm on it. Found a place where I was allocating buffer heads when the
number of requests exceeded 1000 buffer heads, then freeing it back down
to 1000, however, changing this, did not seem to alter the performance
degredation. Will profile and return results.
Jeff
>
> -Andi
>
> -
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