Re: [RFC] [PATCH] cache pollution aware __copy_from_user_ll()

[Hiro Yoshioka]

Thanks for your comments.

On 8/14/05, Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
> On Sun, 2005-08-14 at 18:16 +0900, Hiro Yoshioka wrote:
> > Hi,
> >
> > The following is a patch to reduce a cache pollution
> > of __copy_from_user_ll().
> >
> > When I run simple iozone benchmark to find a performance bottleneck of
> > the linux kernel, I found that __copy_from_user_ll() spent CPU cycle
> > most and it did many cache misses.
> 
> 
> however... you copy something from userspace... aren't you going to USE
> it? The non-termoral versions actually throw the data out of the
> cache... so while this part might be nice, you pay BIG elsewhere....

The oprofile data does not give an evidence that we pay BIG elsewhere.

For examples, the original 2.6.12.4 Top 5 cache misses are the following,

37017 63.4603  vmlinux    __copy_from_user_ll
1049   1.7984  vmlinux    _spin_lock_irqsave
940    1.6115  vmlinux    blk_rq_map_sg
896    1.5361  vmlinux    generic_file_buffered_write
885    1.5172  vmlinux    _spin_lock
pattern9-0-cpu4-0-08141702

cache aware version Top 5 cache misses are
899 5.7305  vmlinux    blk_rq_map_sg
569 3.6270  vmlinux    journal_commit_transaction
531 3.3848  vmlinux    radix_tree_delete
514 3.2764  vmlinux    journal_add_journal_head
505 3.2190  vmlinux    release_pages
...
89 0.5673 vmlinux _mmx_memcpy_nt
pattern9-0-cpu4-0-08141625

What do you think?

Regards,
  Hiro
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