Messages in this thread | | | Subject | Re: [PATCH] mempolicy: convert the shared_policy lock to a rwlock | From | Vlastimil Babka <> | Date | Wed, 18 Nov 2015 14:50:05 +0100 |
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On 11/17/2015 05:17 PM, Nathan Zimmer wrote: > When running the SPECint_rate gcc on some very large boxes it was noticed > that the system was spending lots of time in mpol_shared_policy_lookup. > The gamess benchmark can also show it and is what I mostly used to chase > down the issue since the setup for that I found a easier. > > To be clear the binaries were on tmpfs because of disk I/O reqruirements. > We then used text replication to avoid icache misses and having all the > copies banging on the memory where the instruction code resides. > This results in us hitting a bottle neck in mpol_shared_policy_lookup > since lookup is serialised by the shared_policy lock. > > I have only reproduced this on very large (3k+ cores) boxes. The problem > starts showing up at just a few hundred ranks getting worse until it > threatens to livelock once it gets large enough. > For example on the gamess benchmark at 128 ranks this area consumes only > ~1% of time, at 512 ranks it consumes nearly 13%, and at 2k ranks it is > over 90%. > > To alleviate the contention on this area I converted the spinslock to a > rwlock. This allows the large number of lookups to happen simultaneously. > The results were quite good reducing this to consumtion at max ranks to > around 2%.
At first glance it seems that RCU would be a good fit here and achieve even better lookup scalability, have you considered it?
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