Re: Serious performance degradation on a RAID with kernel 2.6.10-bk7and later

[Bartlomiej ZOLNIERKIEWICZ]

Hi!

> A small update.
>
> Patching mm/filemap.c is not necessary in order to get the improved
> performance!
> It's sufficient to remove roundup_pow_of_two from  |get_init_ra_size ...
>
> So a simple one-liner changes to picture dramatically.
> But why ?!?!?

roundup_pow_of_two() uses fls() and ia64 has buggy fls() implementation
[ seems that David fixed it but patch is not in the mainline yet]:

http://www.mail-archive.com/linux-ia64@xxxxxxxxxxxxxxx/msg01196.html

That would also explain why you couldn't reproduce the problem on ia32 
Xeon machines.

Bartlomiej
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