Messages in this thread | | | Date | Wed, 21 Jan 2009 09:28:24 -0800 | From | Mike Waychison <> | Subject | Re: [PATCH v1 0/8] Deferred dput() and iput() -- reducing lock contention |
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Andi Kleen wrote: > On Tue, Jan 20, 2009 at 10:22:00PM -0800, Mike Waychison wrote: >> Andi Kleen wrote: >>> Mike Waychison <mikew@google.com> writes: >>> >>>> livelock on dcache_lock/inode_lock (specifically in >>>> atomic_dec_and_lock()) >>> I'm not sure how something can livelock in atomic_dec_and_lock which >>> doesn't take a spinlock itself? Are you saying you run into NUMA memory >>> unfairness here? Or did I misparse you? >> By atomic_dec_and_lock, I really meant to say _atomic_dec_and_lock(). > > Ok. So it's basically just the lock that is taken? > > In theory one could likely provide an x86 specific dec-and_lock that > might perform better and doesn't lock if the count is still > 0, but that > would only help if the reference count is still > 0. Is that a common > situation in your test? >
This is precisely what _atomic_dec_and_lock does. It only locks if it looks like the counter will be hitting 0 with this dec.
It is a common case for the count to be > 1, however the troubles we see happen with there are a large number of final dputs (and thus also final iputs)
>> It takes the spinlock if the cmpxchg hidden inside atomic_dec_unless fails. >> >> There are likely NUMA unfairness issues at play, but it's not the main >> worry at this point. >> >>>> This patchset is an attempt to try and reduce the locking overheads >>>> associated >>>> with final dput() and final iput(). This is done by batching dentries and >>>> inodes into per-process queues and processing them in 'parallel' to >>>> consolidate >>>> some of the locking. >>> I was wondering what this does to the latencies when dput/iput >>> is only done for very objects. Does it increase costs then >>> significantly? >> very objects? > > Sorry. > > "is only done for very few objects". Somnhow the few got lost. > Basically latency in the unloaded case. > > I always worry when people do complicated things for the high > load case how the more usual "do it for a single object" workload > fares.
Hmm. I'm not sure. The inline latency should fair out to be better than before on average (common case is to disable pre-emption, enqueue on the postponed list and re-enable pre-emption, which is fast). It's the clearing of the queues that may take a little longer, though the locking overhead should be the same. The trouble is that the queue itself may not be cache hot and that would add a bit of latency.
Would you know of a good way to detect this latency for fewer objects?
> >>> As a high level comment it seems like a lot of work to work >>> around global locks, like the inode_lock, where it might be better to >>> just split the lock up? Mind you I don't have a clear proposal >>> how to do that, but surely it's doable somehow. >>> >> Perhaps.. the only plausible way I can think this would be doable would >> be to rework the global resources (like the global inode_unused LRU list > > One simple way would be to just use multiple lists with an own lock > each. I doubt that would impact the LRU behaviour very much. > >> and deal with inode state transitions), but even then, some sort of >> consistency needs to happen at the super_block level, > > The sb could also look at multiple lists?
Maybe (hashing on inode nr perhaps?).
The other trouble is that dcache_lock which for the most part stands in from of the inode_lock. While developing this patchset, I was originally chasing dcache_lock contention issues. Once I relieved the contention via deferred batching and parallel processing of dput(), I was surprised to see that performance didn't get any better because now I was mostly contending on inode_lock :\
The dcache_lock again could maybe be broken out to be per super_block, but I have no idea how to make that work with mountpoint traversals. Making this guy become a set of smaller locks seems difficult at best.
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