Re: [PATCH RFC V6 1/11] x86/spinlock: replace pv spinlocks withpv ticketlocks
From: Attilio Rao
Date: Wed Mar 21 2012 - 09:04:30 EST
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.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
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