During our testings of 3.8 kernel, we noticed that after the patch
Revert "sched: Update_cfs_shares at period edge" (commit 17bc14b7),
the load between the sockets or larger system can have
large imbalance. For example, for a 4 socket Westmere-EX
(10 cores/socket), we notice the loadings between the sockets
can differ by more than a factor of 4.
We did a simple experiment that kicks off 29 simple
processes that execute a tight loop. We noticed
socket 3 is already starting to schedule on hyperthreaded cpus
(13 loaded cpus) while socket 1 still have lots of
idle cores (3 loaded cpus). Before the patch, the
load was evenly distributed across sockets.
If I turn off CONFIG_SCHED_AUTOGROUP,the loads are also
distributed evenly.
(load on cpus, running on 4)
socket 0 1 2
3
---------------------------------------------------------------------------------------------
cpu: 0-3 0.00 0.00 0.00
99.00
cpu: 4-7 0.00 0.00 0.00
99.20
cpu: 8-11 0.00 0.00 0.00
99.00
cpu: 12-15 99.20 0.00 0.00
0.00
cpu: 16-19 0.00 0.00 0.00
99.00
cpu: 20-23 0.00 0.00 0.00
0.00
cpu: 24-27 0.00 0.00 0.00
0.00
cpu: 28-31 0.00 0.00 0.00
0.00
cpu: 32-35 99.20 0.00 0.00
99.00
cpu: 36-39 99.20 99.40 99.20
0.00
cpu: 40-43 0.00 99.40 99.40
99.20
cpu: 44-47 0.00 99.40 99.40
99.20
cpu: 48-51 99.40 0.00 99.40
99.20
cpu: 52-55 99.20 0.00 99.40
99.20
cpu: 56-59 0.00 0.00 99.40
99.40
cpu: 60-63 0.00 0.00 0.00
99.00
cpu: 64-67 0.00 0.00 0.00
99.40
cpu: 68-71 0.00 0.00 0.00
99.40
cpu: 72-75 99.40 0.00 0.00
0.00
cpu: 76-79 99.40 0.00 0.00
0.00
---------------------------------------------------------------------------------------------
Loaded cpus 7 3 6
13
Is this the intended behavior of sched autogroup? I'm a bit surprised
that we are reserving this much cpu bandwidth for very low load
processes (or interactive processes) in other groups.
So should the sched autogroup config option be turned off by default for server
system, when we are not concerned about interactivity but want to maximize
throughput by balancing out the load?
Thanks for clarifying.
Tim
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