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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