Re: [Xen-devel] [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jun 26, 2013 at 03:52:40PM +0300, Gleb Natapov wrote:
> On Wed, Jun 26, 2013 at 01:37:45PM +0200, Andrew Jones wrote:
> > On Wed, Jun 26, 2013 at 02:15:26PM +0530, Raghavendra K T wrote:
> > > On 06/25/2013 08:20 PM, Andrew Theurer wrote:
> > > >On Sun, 2013-06-02 at 00:51 +0530, Raghavendra K T wrote:
> > > >>This series replaces the existing paravirtualized spinlock mechanism
> > > >>with a paravirtualized ticketlock mechanism. The series provides
> > > >>implementation for both Xen and KVM.
> > > >>
> > > >>Changes in V9:
> > > >>- Changed spin_threshold to 32k to avoid excess halt exits that are
> > > >>    causing undercommit degradation (after PLE handler improvement).
> > > >>- Added  kvm_irq_delivery_to_apic (suggested by Gleb)
> > > >>- Optimized halt exit path to use PLE handler
> > > >>
> > > >>V8 of PVspinlock was posted last year. After Avi's suggestions to look
> > > >>at PLE handler's improvements, various optimizations in PLE handling
> > > >>have been tried.
> > > >
> > > >Sorry for not posting this sooner.  I have tested the v9 pv-ticketlock
> > > >patches in 1x and 2x over-commit with 10-vcpu and 20-vcpu VMs.  I have
> > > >tested these patches with and without PLE, as PLE is still not scalable
> > > >with large VMs.
> > > >
> > > 
> > > Hi Andrew,
> > > 
> > > Thanks for testing.
> > > 
> > > >System: x3850X5, 40 cores, 80 threads
> > > >
> > > >
> > > >1x over-commit with 10-vCPU VMs (8 VMs) all running dbench:
> > > >----------------------------------------------------------
> > > >                                         Total
> > > >Configuration                            Throughput(MB/s)        Notes
> > > >
> > > >3.10-default-ple_on                      22945                   5% CPU 
> > > >in host kernel, 2% spin_lock in guests
> > > >3.10-default-ple_off                     23184                   5% CPU 
> > > >in host kernel, 2% spin_lock in guests
> > > >3.10-pvticket-ple_on                     22895                   5% CPU 
> > > >in host kernel, 2% spin_lock in guests
> > > >3.10-pvticket-ple_off                    23051                   5% CPU 
> > > >in host kernel, 2% spin_lock in guests
> > > >[all 1x results look good here]
> > > 
> > > Yes. The 1x results look too close
> > > 
> > > >
> > > >
> > > >2x over-commit with 10-vCPU VMs (16 VMs) all running dbench:
> > > >-----------------------------------------------------------
> > > >                                         Total
> > > >Configuration                            Throughput              Notes
> > > >
> > > >3.10-default-ple_on                       6287                   55% CPU 
> > > > host kernel, 17% spin_lock in guests
> > > >3.10-default-ple_off                      1849                   2% CPU 
> > > >in host kernel, 95% spin_lock in guests
> > > >3.10-pvticket-ple_on                      6691                   50% CPU 
> > > >in host kernel, 15% spin_lock in guests
> > > >3.10-pvticket-ple_off                    16464                   8% CPU 
> > > >in host kernel, 33% spin_lock in guests
> > > 
> > > I see 6.426% improvement with ple_on
> > > and 161.87% improvement with ple_off. I think this is a very good sign
> > >  for the patches
> > > 
> > > >[PLE hinders pv-ticket improvements, but even with PLE off,
> > > >  we still off from ideal throughput (somewhere >20000)]
> > > >
> > > 
> > > Okay, The ideal throughput you are referring is getting around atleast
> > > 80% of 1x throughput for over-commit. Yes we are still far away from
> > > there.
> > > 
> > > >
> > > >1x over-commit with 20-vCPU VMs (4 VMs) all running dbench:
> > > >----------------------------------------------------------
> > > >                                         Total
> > > >Configuration                            Throughput              Notes
> > > >
> > > >3.10-default-ple_on                      22736                   6% CPU 
> > > >in host kernel, 3% spin_lock in guests
> > > >3.10-default-ple_off                     23377                   5% CPU 
> > > >in host kernel, 3% spin_lock in guests
> > > >3.10-pvticket-ple_on                     22471                   6% CPU 
> > > >in host kernel, 3% spin_lock in guests
> > > >3.10-pvticket-ple_off                    23445                   5% CPU 
> > > >in host kernel, 3% spin_lock in guests
> > > >[1x looking fine here]
> > > >
> > > 
> > > I see ple_off is little better here.
> > > 
> > > >
> > > >2x over-commit with 20-vCPU VMs (8 VMs) all running dbench:
> > > >----------------------------------------------------------
> > > >                                         Total
> > > >Configuration                            Throughput              Notes
> > > >
> > > >3.10-default-ple_on                       1965                   70% CPU 
> > > >in host kernel, 34% spin_lock in guests         
> > > >3.10-default-ple_off                       226                   2% CPU 
> > > >in host kernel, 94% spin_lock in guests
> > > >3.10-pvticket-ple_on                      1942                   70% CPU 
> > > >in host kernel, 35% spin_lock in guests
> > > >3.10-pvticket-ple_off                     8003                   11% CPU 
> > > >in host kernel, 70% spin_lock in guests
> > > >[quite bad all around, but pv-tickets with PLE off the best so far.
> > > >  Still quite a bit off from ideal throughput]
> > > 
> > > This is again a remarkable improvement (307%).
> > > This motivates me to add a patch to disable ple when pvspinlock is on.
> > > probably we can add a hypercall that disables ple in kvm init patch.
> > > but only problem I see is what if the guests are mixed.
> > > 
> > >  (i.e one guest has pvspinlock support but other does not. Host
> > > supports pv)
> > 
> > How about reintroducing the idea to create per-kvm ple_gap,ple_window
> > state. We were headed down that road when considering a dynamic window at
> > one point. Then you can just set a single guest's ple_gap to zero, which
> > would lead to PLE being disabled for that guest. We could also revisit
> > the dynamic window then.
> > 
> Can be done, but lets understand why ple on is such a big problem. Is it
> possible that ple gap and SPIN_THRESHOLD are not tuned properly?

It could be, but it also could be a microcode issue. The earlier version
of Intel (and AMD) CPUs did not have the best detection mechanism and had
a "jitter" to them. The ple gap and ple window values seemed to be choosen
based on microbenchmark - and while they might work great with Windows
type guests - the same is not said about Linux.

In which case if you fiddle with the ple gap/window you might incur
worst performance with Windows guests :-( Or older Linux guests
that use the byte-locking mechanism.

Perhaps the best option is to introduce - as a seperate patchset -
said dynamic window which will be off when pvticket lock is off - and
then based on further CPUs improvements, can turn it on/off?

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