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The venerable menu governor does some things that are quite questionable in my view. First, it includes timer wakeups in the pattern detection data and mixes them up with wakeups from other sources which in some cases causes it to expect what essentially would be a timer wakeup in a time frame in which no timer wakeups are possible (because it knows the time until the next timer event and that is later than the expected wakeup time). Second, it uses the extra exit latency limit based on the predicted idle duration and depending on the number of tasks waiting on I/O, even though those tasks may run on a different CPU when they are woken up. Moreover, the time ranges used by it for the sleep length correction factors depend on whether or not there are tasks waiting on I/O, which again doesn't imply anything in particular, and they are not correlated to the list of available idle states in any way whatever. Also, the pattern detection code in menu may end up considering values that are too large to matter at all, in which cases running it is a waste of time. A major rework of the menu governor would be required to address these issues and the performance of at least some workloads (tuned specifically to the current behavior of the menu governor) is likely to suffer from that. It is thus better to introduce an entirely new governor without them and let everybody use the governor that works better with their actual workloads. The new governor introduced here, the timer events oriented (TEO) governor, uses the same basic strategy as menu: it always tries to find the deepest idle state that can be used in the given conditions. However, it applies a different approach to that problem. First, it doesn't use "correction factors" for the time till the closest timer, but instead it tries to correlate the measured idle duration values with the available idle states and use that information to pick up the idle state that is most likely to "match" the upcoming CPU idle interval. Second, it doesn't take the number of "I/O waiters" into account at all and the pattern detection code in it avoids taking timer wakeups into account. It also only uses idle duration values less than the current time till the closest timer (with the tick excluded) for that purpose. Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org> |
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| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.