Memcached with cfs quota 400% performance boost after bind to 4 cpus

[Wang Jianchao <jianchao.wan9-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi list

I have a test environment with following,
A memcached (memcached -d -m 50000 -u root -p 12301 -c 1000000 -t 16) in cpu cgroup with following config,
cpu.cfs_quota_us = 400000
cpu.cfs_period_us = 100000

And a mutilate loop (mutilate -s x.x.x.x:12301 -T 40 -c 20 -t 60 -W 5 -q 1000000) running on another host
w/o any cgroup config,

When bind memcached to  0-15 with cpuset, 
==========================================
mutilate showed,
#type       avg     std     min     5th    10th    90th    95th    99th
read     1275.8  6358.9    49.8   378.2   418.5   767.2   841.4 53998.5
update      0.0     0.0     0.0     0.0     0.0     0.0     0.0     0.0
op_q        1.0     0.0     1.0     1.0     1.0     1.1     1.1     1.1

Total QPS = 626566.2 (37594133 / 60.0s)

Misses = 0 (0.0%)
Skipped TXs = 0 (0.0%)

RX 9288150851 bytes :  147.6 MB/s
TX 1353390552 bytes :   21.5 MB/s

And perf on memcached showed,
   635,602,955,852      cycles                                                        (30.07%)
   479,554,401,177      instructions              #    0.75  insn per cycle           (40.02%)
    12,585,059,799      L1-dcache-load-misses     #    9.31% of all L1-dcache hits    (50.07%)
   135,140,424,785      L1-dcache-loads                                               (49.96%)
    76,849,156,759      L1-dcache-stores                                              (50.02%)
    45,700,267,543      L1-icache-load-misses                                         (49.97%)
       495,149,862      LLC-load-misses           #   24.96% of all LL-cache hits     (39.95%)
     1,984,134,589      LLC-loads                                                     (39.97%)
       327,130,920      LLC-store-misses                                              (20.06%)
     1,397,111,117      LLC-stores                                                    (20.06%)


When bind memcached to 0-3 with cpuset,
========================================
mutilate showed,
#type       avg     std     min     5th    10th    90th    95th    99th
read      934.7  3669.3    41.1   112.8   129.5   385.3  3321.9 21923.7
update      0.0     0.0     0.0     0.0     0.0     0.0     0.0     0.0
op_q        1.0     0.0     1.0     1.0     1.0     1.1     1.1     1.1

Total QPS = 852885.6 (51173140 / 60.0s)

Misses = 0 (0.0%)
Skipped TXs = 0 (0.0%)

RX 12642165580 bytes :  200.9 MB/s
TX 1842259932 bytes :   29.3 MB/s

And perf on memcached showed,

   621,311,916,151      cycles                                                        (30.01%)
   599,835,965,997      instructions              #    0.97  insn per cycle           (40.02%)
    12,585,889,988      L1-dcache-load-misses     #    7.59% of all L1-dcache hits    (50.00%)
   165,750,518,361      L1-dcache-loads                                               (50.01%)
    93,588,611,989      L1-dcache-stores                                              (50.00%)
    44,445,213,037      L1-icache-load-misses                                         (50.01%)
       568,410,466      LLC-load-misses           #   26.91% of all LL-cache hits     (40.03%)
     2,112,218,392      LLC-loads                                                     (40.00%)
       261,202,604      LLC-store-misses                                              (19.97%)
     1,484,886,714      LLC-stores 


We can see the IPC raised from 0.75 to 0.97, this should be the reason of the performance boost.
What does cause the IPC boost ?

Thanks a million for any help
Jianchao