Re: 2.6.11 vs 2.6.10 slowdown on i686

[Nick Piggin]

Kurt Garloff wrote:
> Hi Nick,

Hi Kurt!

> On Thu, Mar 17, 2005 at 11:37:24PM +1100, Nick Piggin wrote:
>
> > Ian Pratt wrote:
> >
> > > fork: 166 -> 235  (40% slowdown)
> > > exec: 857 -> 1003 (17% slowdown)
> > >
> > > I'm guessing this is down to the 4 level pagetables. This is rather a
> > > surprise as I thought the compiler would optimise most of these
> > > changes away. Apparently not. 
> >
> > There are some changes in the current -bk tree (which are a
> > bit in-flux at the moment) which introduce some optimisations.
> >
> > They should bring 2-level performance close to par with 2.6.10.
> > If not, complain again :)
>
>
> Is there a clean patchset that we should look at to test?

Probably the best thing would be to wait and see what happens
with the ptwalk patches. There is a fix in there for ia64 now,
but I think that may be a temporary one.

Andi is probably keeping an eye on that, but if not then I
could put a patchset together when things finalise in 2.6.

From the profiles I have seen, the ptwalk patches bring page
table walking performance pretty well back to 2.6.10 levels,
however the "aggressive page table freeing" (clear_page_range)
changes that went in at the same time as the 4level stuff
seem to be what is slowing down exit() and unmapping performance.

Not by a huge amount, mind you, and it is not completely wasted
performance, because it provides better page table freeing.
But it is enough to be annoying! I haven't had much time to look
at it lately, but I hope to get onto it soon.

Nick

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