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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(). on each CPU, including cs_dbs_timer() and od_dbs_timer().
WARNING: Please check your CPU specifications to WARNING: Please check your CPU specifications to
make sure that this is safe on your particular system. make sure that this is safe on your particular system.
d. It is not possible to entirely get rid of OS jitter d. As of v3.18, Christoph Lameter's on-demand vmstat workers
from vmstat_update() on CONFIG_SMP=y systems, but you commit prevents OS jitter due to vmstat_update() on
can decrease its frequency by writing a large value CONFIG_SMP=y systems. Before v3.18, is not possible
to /proc/sys/vm/stat_interval. The default value is to entirely get rid of the OS jitter, but you can
HZ, for an interval of one second. Of course, larger decrease its frequency by writing a large value to
values will make your virtual-memory statistics update /proc/sys/vm/stat_interval. The default value is HZ,
more slowly. Of course, you can also run your workload for an interval of one second. Of course, larger values
at a real-time priority, thus preempting vmstat_update(), 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. but if your workload is CPU-bound, this is a bad idea.
However, there is an RFC patch from Christoph Lameter However, there is an RFC patch from Christoph Lameter
(based on an earlier one from Gilad Ben-Yossef) that (based on an earlier one from Gilad Ben-Yossef) that