linux-kernel - Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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Date:	Tue, 25 Jun 2013 09:50:02 -0500
From:	Andrew Theurer <habanero@...ux.vnet.ibm.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, mingo@...hat.com, jeremy@...p.org, x86@...nel.org,
	konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com,
	linux-doc@...r.kernel.org, xen-devel@...ts.xensource.com,
	peterz@...radead.org, mtosatti@...hat.com,
	stefano.stabellini@...citrix.com, andi@...stfloor.org,
	attilio.rao@...rix.com, ouyang@...pitt.edu, gregkh@...e.de,
	agraf@...e.de, chegu_vinod@...com, torvalds@...ux-foundation.org,
	avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, stephan.diestelhorst@....com,
	riel@...hat.com, drjones@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V9 0/19] Paravirtualized ticket spinlocks

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.

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]


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
[PLE hinders pv-ticket improvements, but even with PLE off,
 we still off from ideal throughput (somewhere >20000)]


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]


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]

In summary, I would state that the pv-ticket is an overall win, but the
current PLE handler tends to "get in the way" on these larger guests.

-Andrew

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