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kthread: Add kworker kthreads to OS-jitter documentation 

The kworker workqueue kthreads can also contribute to OS jitter.
The amount of jitter depends on their use, so this commit adds
documentation on avoiding OS jitter due to workqueue use.

Reported-by: Jonathan Clairembault <jonathan.clairembault@novasparks.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2013-04-30 10:48:35 -07:00
parent ce5f4fc861
commit f7bac9b85a
1 changed files with 47 additions and 0 deletions
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47
Documentation/kernel-per-CPU-kthreads.txt
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@ -157,6 +157,53 @@ RCU_SOFTIRQ: Do at least one of the following:
calls and by forcing both kernel threads and interrupts
to execute elsewhere.
Name: kworker/%u:%d%s (cpu, id, priority)
Purpose: Execute workqueue requests
To reduce its OS jitter, do any of the following:
1. Run your workload at a real-time priority, which will allow
preempting the kworker daemons.
2. Do any of the following needed to avoid jitter that your
application cannot tolerate:
a. Build your kernel with CONFIG_SLUB=y rather than
CONFIG_SLAB=y, thus avoiding the slab allocator's periodic
use of each CPU's workqueues to run its cache_reap()
function.
b. Avoid using oprofile, thus avoiding OS jitter from
wq_sync_buffer().
c. Limit your CPU frequency so that a CPU-frequency
governor is not required, possibly enlisting the aid of
special heatsinks or other cooling technologies. If done
correctly, and if you CPU architecture permits, you should
be able to build your kernel with CONFIG_CPU_FREQ=n to
avoid the CPU-frequency governor periodically running
on each CPU, including cs_dbs_timer() and od_dbs_timer().
WARNING: Please check your CPU specifications to
make sure that this is safe on your particular system.
d. It is not possible to entirely get rid of OS jitter
from vmstat_update() on CONFIG_SMP=y systems, but you
can decrease its frequency by writing a large value to
/proc/sys/vm/stat_interval. The default value is HZ,
for an interval of one second. Of course, larger values
will make your virtual-memory statistics update more
slowly. Of course, you can also run your workload at
a real-time priority, thus preempting vmstat_update().
e. If running on high-end powerpc servers, build with
CONFIG_PPC_RTAS_DAEMON=n. This prevents the RTAS
daemon from running on each CPU every second or so.
(This will require editing Kconfig files and will defeat
this platform's RAS functionality.) This avoids jitter
due to the rtas_event_scan() function.
WARNING: Please check your CPU specifications to
make sure that this is safe on your particular system.
f. If running on Cell Processor, build your kernel with
CBE_CPUFREQ_SPU_GOVERNOR=n to avoid OS jitter from
spu_gov_work().
WARNING: Please check your CPU specifications to
make sure that this is safe on your particular system.
g. If running on PowerMAC, build your kernel with
CONFIG_PMAC_RACKMETER=n to disable the CPU-meter,
avoiding OS jitter from rackmeter_do_timer().
Name: rcuc/%u
Purpose: Execute RCU callbacks in CONFIG_RCU_BOOST=y kernels.
To reduce its OS jitter, do at least one of the following: