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SubjectRe: [PATCH] locking/pvqspinlock: Hybrid PV queued/unfair locks
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On 03/11/17 16:35, Waiman Long wrote:
> Currently, all the lock waiters entering the slowpath will do one
> lock stealing attempt to acquire the lock. That helps performance,
> especially in VMs with over-committed vCPUs. However, the current
> pvqspinlocks still don't perform as good as unfair locks in many cases.
> On the other hands, unfair locks do have the problem of lock starvation
> that pvqspinlocks don't have.
>
> This patch combines the best attributes of an unfair lock and a
> pvqspinlock into a hybrid lock with 2 modes - queued mode & unfair
> mode. A lock waiter goes into the unfair mode when there are waiters
> in the wait queue but the pending bit isn't set. Otherwise, it will
> go into the queued mode waiting in the queue for its turn.
>
> On a 2-socket 36-core E5-2699 v3 system (HT off), a kernel build
> (make -j<n>) was done in a VM with unpinned vCPUs 3 times with the
> best time selected and <n> is the number of vCPUs available. The build
> times of the original pvqspinlock, hybrid pvqspinlock and unfair lock
> with various number of vCPUs are as follows:
>
> vCPUs pvqlock hybrid pvqlock unfair lock
> ----- ------- -------------- -----------
> 30 342.1s 329.1s 329.1s
> 36 314.1s 305.3s 307.3s
> 45 345.0s 302.1s 306.6s
> 54 365.4s 308.6s 307.8s
> 72 358.9s 293.6s 303.9s
> 108 343.0s 285.9s 304.2s
>
> The hybrid pvqspinlock performs better or comparable to the unfair
> lock.
>
> By turning on QUEUED_LOCK_STAT, the table below showed the number
> of lock acquisitions in unfair mode and queue mode after a kernel
> build with various number of vCPUs.
>
> vCPUs queued mode unfair mode
> ----- ----------- -----------
> 30 9,130,518 294,954
> 36 10,856,614 386,809
> 45 8,467,264 11,475,373
> 54 6,409,987 19,670,855
> 72 4,782,063 25,712,180
>
> It can be seen that as the VM became more and more over-committed,
> the ratio of locks acquired in unfair mode increases. This is all
> done automatically to get the best overall performance as possible.
>
> The table below shows the kernel build times on a smaller 2-socket
> 16-core 32-thread E5-2620 v4 system.
>
> vCPUs pvqlock hybrid pvqlock unfair lock
> ----- ------- -------------- -----------
> 16 436.8s 433.4s 435.6s
> 36 366.2s 364.8s 364.5s
> 48 423.6s 376.3s 370.2s
> 64 433.1s 376.6s 376.8s
>
> Again, the performance of the hybrid pvqspinlock was comparable to
> that of the unfair lock.
>
> Signed-off-by: Waiman Long <longman@redhat.com>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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