Linux Virtualization - Mar 2014 - [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a PV guest

On Thu, Mar 13, 2014 at 02:16:06PM +0100, Paolo Bonzini
wrote:
> Il 13/03/2014 11:54, David Vrabel ha scritto:
> >On 12/03/14 18:54, Waiman Long wrote:
> >>Locking is always an issue in a virtualized environment as the
virtual
> >>CPU that is waiting on a lock may get scheduled out and hence block
> >>any progress in lock acquisition even when the lock has been freed.
> >>
> >>One solution to this problem is to allow unfair lock in a
> >>para-virtualized environment. In this case, a new lock acquirer can
> >>come and steal the lock if the next-in-line CPU to get the lock is
> >>scheduled out. Unfair lock in a native environment is generally not
a
> >>good idea as there is a possibility of lock starvation for a
heavily
> >>contended lock.
> >
> >I do not think this is a good idea -- the problems with unfair locks
are
> >worse in a virtualized guest.  If a waiting VCPU deschedules and has to
> >be kicked to grab a lock then it is very likely to lose a race with
> >another running VCPU trying to take a lock (since it takes time for the
> >VCPU to be rescheduled).
> 
> Actually, I think the unfair version should be automatically
> selected if running on a hypervisor.  Per-hypervisor pvops can
> choose to enable the fair one.
> 
> Lock unfairness may be particularly evident on a virtualized guest
> when the host is overcommitted, but problems with fair locks are
> even worse.
> 
> In fact, RHEL/CentOS 6 already uses unfair locks if
> X86_FEATURE_HYPERVISOR is set.  The patch was rejected upstream in
> favor of pv ticketlocks, but pv ticketlocks do not cover all
> hypervisors so perhaps we could revisit that choice.
> 
> Measurements were done by Gleb for two guests running 2.6.32 with 16
> vcpus each, on a 16-core system.  One guest ran with unfair locks,
> one guest ran with fair locks.  Two kernel compilations ("time make
And when you say fair locks are you saying PV ticketlocks or generic
ticketlocks? 
> -j 16 all") were started at the same time on both guests, and times
> were as follows:
> 
>     unfair:                         fair:
>     real 13m34.674s                 real 19m35.827s
>     user 96m2.638s                  user 102m38.665s
>     sys 56m14.991s                  sys 158m22.470s
> 
>     real 13m3.768s                  real 19m4.375s
>     user 95m34.509s                 user 111m9.903s
>     sys 53m40.550s                  sys 141m59.370s
> 
> Actually, interpreting the numbers shows an even worse slowdown.
> 
> Compilation took ~6.5 minutes in a guest when the host was not
> overcommitted, and with unfair locks everything scaled just fine.
You should see the same values with the PV ticketlock. It is not clear
to me if this testing did include that variant of locks?
> 
> Ticketlocks fell completely apart; during the first 13 minutes they
> were allotted 16*6.5=104 minutes of CPU time, and they spent almost
> all of it spinning in the kernel (102 minutes in the first run).
Right, the non-PV variant of them do fall apart. That is why
PV ticketlocks are so nice.
> They did perhaps 30 seconds worth of work because, as soon as the
> unfair-lock guest finished and the host was no longer overcommitted,
> compilation finished in 6 minutes.
> 
> So that's approximately 12x slowdown from using non-pv fair locks
> (vs. unfair locks) on a 200%-overcommitted host.
Ah, so it was non-PV.

I am curious if the test was any different if you tested PV ticketlocks
vs Red Hat variant of unfair locks.
> 
> Paolo
> 
> >>With the unfair locking activated on bare metal 4-socket
Westmere-EX
> >>box, the execution times (in ms) of a spinlock micro-benchmark were
> >>as follows:
> >>
> >>  # of    Ticket       Fair	    Unfair
> >>  tasks    lock     queue lock    queue lock
> >>  ------  -------   ----------    ----------
> >>    1       135        135	     137
> >>    2      1045       1120	     747
> >>    3      1827       2345     	    1084
> >>    4      2689       2934	    1438
> >>    5      3736       3658	    1722
> >>    6      4942       4434	    2092
> >>    7      6304       5176          2245
> >>    8      7736       5955          2388
> >
> >Are these figures with or without the later PV support patches?
> 
>

Il 17/03/2014 20:05, Konrad Rzeszutek Wilk ha scritto:
>> > Measurements were done by Gleb for two guests running 2.6.32 with
16
>> > vcpus each, on a 16-core system.  One guest ran with unfair locks,
>> > one guest ran with fair locks.  Two kernel compilations
("time make
> And when you say fair locks are you saying PV ticketlocks or generic
> ticketlocks?
Generic, of course.
> You should see the same values with the PV ticketlock. It is not clear
> to me if this testing did include that variant of locks?
Yes, PV is fine.  But up to this point of the series, we are concerned 
about spinlock performance when running on an overcommitted hypervisor 
that doesn't support PV spinlocks.

Paolo

On 03/18/2014 04:14 AM, Paolo Bonzini wrote:
> Il 17/03/2014 20:05, Konrad Rzeszutek Wilk ha scritto:
>>> > Measurements were done by Gleb for two guests running 2.6.32
with 16
>>> > vcpus each, on a 16-core system.  One guest ran with unfair
locks,
>>> > one guest ran with fair locks.  Two kernel compilations
("time make
>> And when you say fair locks are you saying PV ticketlocks or generic
>> ticketlocks?
>
> Generic, of course.
>
>> You should see the same values with the PV ticketlock. It is not clear
>> to me if this testing did include that variant of locks?
>
> Yes, PV is fine.  But up to this point of the series, we are concerned 
> about spinlock performance when running on an overcommitted hypervisor 
> that doesn't support PV spinlocks.
The unfair queue lock is designed in such a way that it will only be 
activated when running in a PV guest or it won't be mergeable upstream. 
So there must be some way to determine if it is running under a hypervisor.

-Longman