Messages in this thread | | | Date | Thu, 05 Jun 2008 19:43:00 +1000 | From | npiggin@suse ... | Subject | [patch 0/7] speculative page references, lockless pagecache, lockless gup |
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Hi,
I've decided to submit the speculative page references patch to get merged. I think I've now got enough reasons to get it merged. Well... I always thought I did, I just didn't think anyone else thought I did. If you know what I mean.
cc'ing the powerpc guys specifically because everyone else who probably cares should be on linux-mm...
So speculative page references are required to support lockless pagecache and lockless get_user_pages (on architectures that can't use the x86 trick). Other uses for speculative page references could also pop up, it is a pretty useful concept. Doesn't need to be pagecache pages either.
Anyway,
lockless pagecache: - speeds up single threaded pagecache lookup operations significantly, by avoiding atomic operations, memory barriers, and interrupts-off sections. I just measured again on a few CPUs I have lying around here, and the speedup is over 2x reduction in cycles on them all, closer to 3x in some cases.
find_get_page takes: ppc970 (g5) K10 P4 Nocona Core2 vanilla 275 (cycles) 85 315 143 lockless 125 40 127 61
- speeds up single threaded pagecache modification operations, by using regular spinlocks rather than rwlocks and avoiding an atomic operation on x86 for one. Also, most real paths which involve pagecache modification also involve pagecache lookups, so it is hard not to get a net speedup.
- solves the rwlock starvation problem for pagecache operations. This is being noticed on big SGI systems, but theoretically could happen on relatively small systems (dozens of CPUs) due to the really nasty writer starvation problem of rwlocks -- not even hardware fairness can solve that.
- improves pagecache scalability to operations on a single file. I demonstrated page faults to a single file were improved in throughput by 250x on a 64-way Altix several years ago. We now have systems with thousands of CPUs in them.
lockless get_user_pages: - provides a way to operate on user pages which is scalable to many threads, and does not get impacted by, or contribute to, mmap_sem contention.
- Alrady shown to speed up DB2 running OLTP by a significant amount.
The speculative page references idea has been out there for quite a few years now, and never been disproved.
So, that's the jist of my justification. If it were up to me, then I would have merged the thing solely on the very first point under lockless pagecache, but...
Review/comments/testing appreciated. I wonder how people feel about merging this soon?
(the actual patchset must go on top of the fast get_user_pages patches I posted earlier because I'm adding the powerpc variant of that here)
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