Messages in this thread | | | Date | Thu, 7 Jan 2010 08:19:56 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [RFC][PATCH 6/8] mm: handle_speculative_fault() |
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On Thu, 7 Jan 2010, Christoph Lameter wrote: > > > > depends on the workload; on a many-threads-java workload, you also get > > it for write quite a bit (lots of malloc/frees in userspace in addition > > to pagefaults).. at which point you do end up serializing on the > > zeroing. > > > > There's some real life real big workloads that show this pretty badly; > > so far the workaround is to have glibc batch up a lot of the free()s.. > > but that's just pushing it a little further out. > > Again mmap_sem is a rwsem and only a read lock is held. Zeroing in > do_anonymous_page can occur concurrently on multiple processors in the > same address space. The pte lock is intentionally taken *after* zeroing to > allow concurrent zeroing to occur.
You're missing what Arjan said - the jav workload does a lot of memory allocations too, causing mmap/munmap.
So now some paths are indeed holding it for writing (or need to wait for it to become writable). And the fairness of rwsems quite possibly then impacts throughput a _lot_..
(Side note: I wonder if we should wake up _all_ readers when we wake up any. Right now, we wake up all readers - but only until we hit a writer. Which is the _fair_ thing to do, but it does mean that we can end up in horrible patterns of alternating readers/writers, when it could be much better to just say "release the hounds" and let all pending readers go after a writer has had its turn).
Linus
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