svn commit: r46969 - head/en_US.ISO8859-1/htdocs/news/status
Warren Block <[email protected]>
| Newsgroups | gmane.os.freebsd.devel.cvs.doc |
|---|---|
| Message-ID | <[email protected]> |
Author: wblock Date: Tue Jul 14 15:08:58 2015 New Revision: 46969 URL: https://svnweb.freebsd.org/changeset/doc/46969 Log: Add Konstantin Belousov <[email protected]>'s Cx state report. Modified: head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml Modified: head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml ============================================================================== --- head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml Tue Jul 14 14:20:28 2015 (r46968) +++ head/en_US.ISO8859-1/htdocs/news/status/report-2015-04-2015-06.xml Tue Jul 14 15:08:58 2015 (r46969) @@ -971,4 +971,86 @@ </task> </help> </project> + + <project cat='kern'> + <title>Sleep States Enhancements on <tt>x86</tt></title> + + <contact> + <person> + <name> + <given>Konstantin</given> + <common>Belousov</common> + </name> + <email>[email protected]</email> + </person> + </contact> + + <links> + <url href="https://svnweb.freebsd.org/base?view=revision&revision=282678" /> + </links> + + <body> + <p>The ACPI specication defines CPU Cx states, which are idle + states. Methods to enter the state and miscellaneous + information like state leave latency are returned by the _CST + ACPI method. To save energy and reduce useless heating, the + operating system enters the Cx state when the CPU has no work + to do. C0 is the non-idle state, while C1, C2, and C3 + (defined by ACPI) each represent an idle state with + sequentially more energy saving, but also with higher latency + of leave and possibly greater secondary costs. For example, + C1 is entered by executing the HLT instruction and has no + architecturally visible side effects, while entering C3 drops + the CPU cache and usually requires special chipset programming + to correctly handle requests from I/O devices to the CPU. Do + not confuse Cx, Px and Sx: Cx states are only meaningful when + the system is in fully operational state S0; Px states are + only meaningful when the system is not in the idle state, + C0.</p> + + <p>Modern Intel CPUs enter Cx (x >= 1) states with the + dedicated instruction MWAIT, which enters a specified + low-power state until a specific write is observed by the CPU + bus logic. There is a complimentary MONITOR instruction to + set the monitored bus address. The legacy port I/O method of + entering Cx state is emulated by CPU microcode, which + intercepts the port I/O and executes MWAIT internally. Using + MWAIT as the method of entering Cx requires following + processor-specific procedures, which are communicated to the + operating system by the vendor-specific extensions in _CST. + The operating system must indicate readiness to support MWAIT + when calling _CST. Claimed benefits of using MWAIT are reduced + latencies of leaving the idle state, and visibility of more + deep states than defined by the common ACPI specification. + Still, modern Intel platforms report deep states as C2 to + avoid the not needed bus-mastering avoidance.</p> + + <p>The new code asks ACPI for the Intel vendor-specific _CST + extensions, parses them, and uses MWAIT Cx entrance methods + when available. The change was committed as r282678 to + HEAD.</p> + + <p>For Linux, Intel provides a driver which does not depend on + the ACPI tables to use MWAIT for entering Cx states. For all + Intel CPUs after Core2, the driver contains the description of + the Cx mode latencies and quirks, eliminating dependency on + the correct BIOS information, which is often incorrect. The + approach of porting the Linux driver was considered by several + people, but all evaluators independently concluded that the + project cannot maintain such an approach without direct + involvement from Intel.</p> + + <p>During the work, around 500 lines of identical code between + the i386 and amd64 version of the idle handling were moved to + the common location <tt>x86/x86/cpu_machdep.c</tt>. Now the + i386 and amd64 <tt>machdep.c</tt> files contain only unique + machine-dependent routines. This advance depended on John + Baldwin's elimination of the unmaintained Xen PVM i386 + port.</p> + </body> + + <sponsor> + The FreeBSD Foundation + </sponsor> + </project> </report> _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/svn-doc-all To unsubscribe, send any mail to "[email protected]"