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Date: Tue, 11 Feb 2014 13:17:37 -0500
From: Waiman Long <waiman.long@...com>
To: Peter Zijlstra <peterz@...radead.org>
CC: linux-kernel@...r.kernel.org, Jason Low <jason.low2@...com>,
mingo@...nel.org, paulmck@...ux.vnet.ibm.com,
torvalds@...ux-foundation.org, tglx@...utronix.de, riel@...hat.com,
akpm@...ux-foundation.org, davidlohr@...com, hpa@...or.com,
andi@...stfloor.org, aswin@...com, scott.norton@...com,
chegu_vinod@...com
Subject: Re: [PATCH 7/8] locking: Introduce qrwlock
Peter,
I had written a test program to repetitively take a single rwlock with
programmable number of threads and count their execution times. Each
thread takes the lock 5M times on a 4-socket 40-core Westmere-EX
system. I bound all the threads to different CPUs with the following
3 configurations:
1) Both CPUs and lock are in the same node
2) CPUs and lock are in different nodes
3) Half of the CPUs are in same node as the lock & the other half
are remote
The results of the test run are as follows (execution times in ms,
smaller is better, qrwlock uses MCS lock for queuing):
R:W ratio = 0:1 (write lock only)
---------------------------------
# of rwlock mean/std qrwlock mean/std
threads Local , Remote Local , Remote
------- --------------------- ----------------------
1 75/ 0, 76/ 0 44/ 0, 44/ 0
2 924/ 6, 818/ 11 233/ 22, 685/ 1
3 1680/ 67, 1676/ 50 422/ 24, 640/305
4 1727/ 402, 1661/ 620 563/ 85, 694/384
5 2634/ 861, 2746/ 426 787/276, 829/401
6 2969/1196, 2892/ 398 810/307, 968/451
7 3532/1512, 3137/1109 936/387, 1060/512
8 3979/1698, 4190/1526 1134/590, 1120/503
R:W ratio = 1:1
---------------
# of rwlock mean/std qrwlock mean/std
threads Local , Remote Local , Remote
------- --------------------- ----------------------
1 82/ 0, 81/ 0 65/ 0, 65/ 0
2 844/ 6, 829/ 1 752/ 4, 894/ 2
3 1683/ 65, 1628/ 29 980/ 14, 1046/ 24
4 2661/ 65, 1747/ 609 1356/ 16, 1405/ 13
5 2227/ 868, 2887/ 395 1625/ 32, 1679/ 29
6 2553/ 968, 3924/1319 1935/ 18, 1986/ 65
7 3392/1325, 3674/1430 2302/ 41, 2313/ 83
8 3822/1545, 5163/2103 2654/ 31, 2654/ 67
R:W ratio = 10:1
----------------
# of rwlock mean/std qrwlock mean/std
threads Local , Remote Local , Remote
------- --------------------- ----------------------
1 86/ 0, 86/ 0 83/ 0, 83/ 0
2 656/ 2, 640/ 3 546/ 2, 635/ 1
3 1154/ 39, 1053/ 64 934/ 27, 968/ 31
4 1810/ 26, 1535/ 399 1357/ 34, 1382/ 52
5 1994/ 495, 2618/ 27 1713/ 52, 1785/ 47
6 2371/ 488, 3099/ 105 2093/ 80, 2131/ 77
7 2846/ 751, 3068/ 689 2495/103, 2527/107
8 3392/1009, 3566/1017 2899/130, 2928/126
For reference, I also made the same measurement for ticket spinlock with
essentially no variation in execution time between different threads:
# of spinlock mean
threads Local, Remote
------- -------------
1 70, 70
2 757, 779
3 1892, 1858
4 2818, 2794
5 3829, 3851
6 4957, 5069
7 6209, 6251
8 7524, 7564
In summary,
1) The qrwlock is always faster than the rwlock with the exception
of about 8% slowdown with 1:1 read-write lock ratio and 2 contending
threads on a remote lock. In the most common case of 1-socket
system, qrwlock will not be slower than rwlock.
2) qrwlock has much less variation in execution times (more consistent
performance) when compared with rwlock.
3) For rwlock with 2-3 contending threads, remote lock actually
performs slightly better than local lock.
4) For qrwlock, local lock performs slightly better than remote lock
with less variation in execution time.
As for the use of ticket lock for queuing purpose, the spinlock
performance figures above seem to suggest that it will perform worse
than MCS lock with even a few contending threads. This is especially
a problem with contending threads coming from different nodes.
Below are the performance data with half local and half remote CPUs:
# of threads = 8
R/W ratio rwlock mean/std qrwlock mean/std
--------- --------------- ----------------
0:1 3476/868 1304/484
1:1 3452/975 2984/777
10:1 3215/914 2963/107
The rwlock actually performs slightly better when CPUs have different access
times to the lock. The qrwlock, on the other hand, performs slightly worse
with much larger variation in execution times.
In the case of ticket spinlock with asymmetric access time (half
local CPUs, half remote CPUs), the performance data were:
# of threads spinlock mean
------------ -------------
2 4570
4 9927
6 15197
8 20573
Ticket spinlock seems to be 3-5X slower with asymmetric access time. So
it may not be such a good idea to replace the MCS lock by a ticket
lock in qrwlock. I will take some additional measurement with your patch
to see how it perform.
-Longman
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