Messages in this thread | | | Date | Tue, 23 Apr 2019 05:41:48 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH v12 00/31] Speculative page faults |
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On Tue, Apr 23, 2019 at 12:47:07PM +0200, Michal Hocko wrote: > On Mon 22-04-19 14:29:16, Michel Lespinasse wrote: > [...] > > I want to add a note about mmap_sem. In the past there has been > > discussions about replacing it with an interval lock, but these never > > went anywhere because, mostly, of the fact that such mechanisms were > > too expensive to use in the page fault path. I think adding the spf > > mechanism would invite us to revisit this issue - interval locks may > > be a great way to avoid blocking between unrelated mmap_sem writers > > (for example, do not delay stack creation for new threads while a > > large mmap or munmap may be going on), and probably also to handle > > mmap_sem readers that can't easily use the spf mechanism (for example, > > gup callers which make use of the returned vmas). But again that is a > > separate topic to explore which doesn't have to get resolved before > > spf goes in. > > Well, I believe we should _really_ re-evaluate the range locking sooner > rather than later. Why? Because it looks like the most straightforward > approach to the mmap_sem contention for most usecases I have heard of > (mostly a mm{unm}ap, mremap standing in the way of page faults). > On a plus side it also makes us think about the current mmap (ab)users > which should lead to an overall code improvements and maintainability.
Dave Chinner recently did evaluate the range lock for solving a problem in XFS and didn't like what he saw:
https://lore.kernel.org/linux-fsdevel/20190418031013.GX29573@dread.disaster.area/T/#md981b32c12a2557a2dd0f79ad41d6c8df1f6f27c
I think scaling the lock needs to be tied to the actual data structure and not have a second tree on-the-side to fake-scale the locking. Anyway, we're going to have a session on this at LSFMM, right?
> SPF sounds like a good idea but it is a really big and intrusive surgery > to the #PF path. And more importantly without any real world usecase > numbers which would justify this. That being said I am not opposed to > this change I just think it is a large hammer while we haven't seen > attempts to tackle problems in a simpler way.
I don't think the "no real world usecase numbers" is fair. Laurent quoted:
> Ebizzy: > ------- > The test is counting the number of records per second it can manage, the > higher is the best. I run it like this 'ebizzy -mTt <nrcpus>'. To get > consistent result I repeated the test 100 times and measure the average > result. The number is the record processes per second, the higher is the best. > > BASE SPF delta > 24 CPUs x86 5492.69 9383.07 70.83% > 1024 CPUS P8 VM 8476.74 17144.38 102%
and cited 30% improvement for you-know-what product from an earlier version of the patch.
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