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documentation: Update based on on-demand vmstat workers

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Now that the on-demand vmstat workers commit is in mainline, it is
possible to eliminate vmstat_update()-induced OS jitter.  This commit
updates the documentation accordingly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2015-01-24 22:24:14 -08:00
parent d2af1ad73e
commit 89bf5d82ed
1 changed files with 10 additions and 8 deletions
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18
Documentation/kernel-per-CPU-kthreads.txt
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@ -190,14 +190,16 @@ To reduce its OS jitter, do any of the following:
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(),
d. As of v3.18, Christoph Lameter's on-demand vmstat workers
commit prevents OS jitter due to vmstat_update() on
CONFIG_SMP=y systems. Before v3.18, is not possible
to entirely get rid of the OS jitter, 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(),
but if your workload is CPU-bound, this is a bad idea.
However, there is an RFC patch from Christoph Lameter
(based on an earlier one from Gilad Ben-Yossef) that