Re: [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks with pv ticketlocks
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On 21/03/12 10:20, Raghavendra K T wrote:
From: Jeremy Fitzhardinge<jeremy.fitzhardinge@xxxxxxxxxx> 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.patchFor 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_scriptHere 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.benchAs 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_scriptHere 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.benchAs 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. == ResultsThis 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization