Re: [Xen-devel] [PATCHv2 0/3] Implement per-cpu reader-writer locks

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 20/11/15 16:03, Malcolm Crossley wrote:
> This patch series adds per-cpu reader-writer locks as a generic lock
> implementation and then converts the grant table and p2m rwlocks to
> use the percpu rwlocks, in order to improve multi-socket host performance.
> 
> CPU profiling has revealed the rwlocks themselves suffer from severe cache
> line bouncing due to the cmpxchg operation used even when taking a read lock.
> Multiqueue paravirtualised I/O results in heavy contention of the grant table
> and p2m read locks of a specific domain and so I/O throughput is bottlenecked
> by the overhead of the cache line bouncing itself.
> 
> Per-cpu read locks avoid lock cache line bouncing by using a per-cpu data
> area to record a CPU has taken the read lock. Correctness is enforced for the 
> write lock by using a per lock barrier which forces the per-cpu read lock 
> to revert to using a standard read lock. The write lock then polls all 
> the percpu data area until active readers for the lock have exited.
> 
> Removing the cache line bouncing on a multi-socket Haswell-EP system 
> dramatically improves performance, with 16 vCPU network IO performance going 
> from 15 gb/s to 64 gb/s! The host under test was fully utilising all 40 
> logical CPU's at 64 gb/s, so a bigger logical CPU host may see an even better
> IO improvement.

Impressive -- thanks for doing this work.

One question: Your description here sounds like you've tested with a
single large domain, but what happens with multiple domains?

It looks like the "per-cpu-rwlock" is shared by *all* locks of a
particular type (e.g., all domains share the per-cpu p2m rwlock).
(Correct me if I'm wrong here.)

Which means two things:

 1) *Any* writer will have to wait for the rwlock of that type to be
released on *all* domains before being able to write.  Is there any risk
that on a busy system, this will be an unusually long wait?

2) *All* domains will have to take the slow path for reading when a
*any* domain has or is trying to acquire the write lock.  What is the
probability that on a busy system that turns out to be "most of the time"?

#2 is of course no worse than it is now, but #1 could be a bit of a bear.

 -George


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