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Date: Wed, 21 Mar 2012 13:04:25 +0000 From: Attilio Rao <attilio.rao@...rix.com> To: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> CC: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, the arch/x86 maintainers <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>, Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>, KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>, Xen Devel <xen-devel@...ts.xensource.com>, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, Virtualization <virtualization@...ts.linux-foundation.org>, Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>, Stephan Diestelhorst <stephan.diestelhorst@....com>, Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>, Stefano Stabellini <Stefano.Stabellini@...citrix.com> Subject: Re: [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks with pv ticketlocks On 21/03/12 10:20, Raghavendra K T wrote: > From: Jeremy Fitzhardinge<jeremy.fitzhardinge@...rix.com> > > Rather than outright replacing the entire spinlock implementation in > order to paravirtualize it, keep the ticket lock implementation but add > a couple of pvops hooks on the slow patch (long spin on lock, unlocking > a contended lock). > > Ticket locks have a number of nice properties, but they also have some > surprising behaviours in virtual environments. They enforce a strict > FIFO ordering on cpus trying to take a lock; however, if the hypervisor > scheduler does not schedule the cpus in the correct order, the system can > waste a huge amount of time spinning until the next cpu can take the lock. > > (See Thomas Friebel's talk "Prevent Guests from Spinning Around" > http://www.xen.org/files/xensummitboston08/LHP.pdf for more details.) > > To address this, we add two hooks: > - __ticket_spin_lock which is called after the cpu has been > spinning on the lock for a significant number of iterations but has > failed to take the lock (presumably because the cpu holding the lock > has been descheduled). The lock_spinning pvop is expected to block > the cpu until it has been kicked by the current lock holder. > - __ticket_spin_unlock, which on releasing a contended lock > (there are more cpus with tail tickets), it looks to see if the next > cpu is blocked and wakes it if so. > > When compiled with CONFIG_PARAVIRT_SPINLOCKS disabled, a set of stub > functions causes all the extra code to go away. > I've made some real world benchmarks based on this serie of patches applied on top of a vanilla Linux-3.3-rc6 (commit 4704fe65e55fb088fbcb1dc0b15ff7cc8bff3685), with both CONFIG_PARAVIRT_SPINLOCK=y and n, which means essentially 4 versions compared: * vanilla - CONFIG_PARAVIRT_SPINLOCK - patch * vanilla + CONFIG_PARAVIRT_SPINLOCK - patch * vanilla - CONFIG_PARAVIRT_SPINLOCK + patch * vanilla + CONFIG_PARAVIRT_SPINLOCK + patch (you can check out the monolithic kernel configuration I used, and verify the sole difference, here): http://xenbits.xen.org/people/attilio/jeremy-spinlock/kernel-configs/ Tests, information and results are summarized below. == System used information: * Machine is a XEON x3450, 2.6GHz, 8-ways system: http://xenbits.xen.org/people/attilio/jeremy-spinlock/dmesg * System version, a Debian Squeeze 6.0.4: http://xenbits.xen.org/people/attilio/jeremy-spinlock/debian-version * gcc version, 4.4.5: http://xenbits.xen.org/people/attilio/jeremy-spinlock/gcc-version == Tests performed * pgbench based on PostgreSQL 9.2 (development version) as it has a lot of scalability improvements in it: http://www.postgresql.org/docs/devel/static/install-getsource.html I used a stock installation, with only this simple configuration change: http://xenbits.xen.org/people/attilio/jeremy-spinlock/postsgresql.conf.patch For collecting data I used this simple scripts, which runs the test 10 times every time with a different set of threads (from 1 to 64). Please note that the first 8 runs cache all the data in memory in order to avoid subsequent I/O, thus they are discarded in sampling and calculation: http://xenbits.xen.org/people/attilio/jeremy-spinlock/pgbench_script Here is the crude data (please remind this is tps, thus the higher the better): http://xenbits.xen.org/people/attilio/jeremy-spinlock/pgbench-crude-datas/ And here are data chartered with ministat tool, comparing all the 4 kernel configuration for every thread configuration: http://xenbits.xen.org/people/attilio/jeremy-spinlock/pgbench-9.2-total.bench As you can see, the patch doesn't really show a statistically meaningful difference for this workload, excluding the single-thread run for the patched + CONFIG_PARAVIRT_SPINLOCK=y case, which seems 5% faster. * pbzip2, which is a parallel version of bzip2, supposed to reproduce a CPU-intensive, multithreaded, application. The file choosen for compression is 1GB sized, got from /dev/urandom (this is not published but I may have it, so if you need it for more tests please just ask), and all the I/O is done on a tmpfs volume in order to avoid I/O floaty effects. For collecting data I used this simple scripts, which runs the test 10 times every time with a different set of threads (from 1 to 64): http://xenbits.xen.org/people/attilio/jeremy-spinlock/pbzip2bench_script Here is the crude data (please remind this is time(1) output, thus the lower the better): http://xenbits.xen.org/people/attilio/jeremy-spinlock/pbzip2-crude-datas/ And here are data chartered with ministat tool, comparing all the 4 kernel configuration for every thread configuration: http://xenbits.xen.org/people/attilio/jeremy-spinlock/pbzip2-1.1.1-total.bench As you can see, the patch doesn't really show a statistically meaningful difference for this workload. * kernbench-0.50 run, doing I/O on a 10GB tmpfs volume (thus no actual I/O involved), with the following invokation: ./kernbench -n10 -s -c16 -M -f (I had to do that because kernbench wasn't getting a good maximum value at all, thus I disabled default maximum and forced for 16 threads). Here is the crude data (please remind this is time(1) output, thus the lower the better): http://xenbits.xen.org/people/attilio/jeremy-spinlock/kernbench-crude-datas/ Please note that kernbench already calculates std deviation for them. However I also wanted a ministat summary in order to quickly display any possible difference, thus I just replicated 3 times any value (the minimum requested by ministat) and charted them: http://xenbits.xen.org/people/attilio/jeremy-spinlock/kernbench-0.50-total.bench Again, it doesn't seem to be any meaningful statistical difference. == Results This test points in the direction that Jeremy's rebased patches don't introduce a peformance penalty at all, but also that we could likely consider CONFIG_PARAVIRT_SPINLOCK option removal, or turn it on by default and suggest disabling just on very old CPUs (assuming a performance regression can be proven there). If you have questions please let me know. Thanks, Attilio -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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