Re: [PATCH] mempolicy: convert the shared_policy lock to a rwlock

[Vlastimil Babka]

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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