svn commit: r49563 - head/en_US.ISO8859-1/htdocs/news/status
Benjamin Kaduk <[email protected]>
| Newsgroups | gmane.os.freebsd.devel.cvs.doc |
|---|---|
| Message-ID | <[email protected]> |
Author: bjk
Date: Mon Oct 24 04:28:40 2016
New Revision: 49563
URL: https://svnweb.freebsd.org/changeset/doc/49563
Log:
Add entry on timekeeping code improvements from kib
Modified:
head/en_US.ISO8859-1/htdocs/news/status/report-2016-07-2016-09.xml
Modified: head/en_US.ISO8859-1/htdocs/news/status/report-2016-07-2016-09.xml
==============================================================================
--- head/en_US.ISO8859-1/htdocs/news/status/report-2016-07-2016-09.xml Mon Oct 24 03:58:13 2016 (r49562)
+++ head/en_US.ISO8859-1/htdocs/news/status/report-2016-07-2016-09.xml Mon Oct 24 04:28:40 2016 (r49563)
@@ -1358,4 +1358,70 @@
can.</p>
</body>
</project>
+
+ <project cat='kern'>
+ <title>Timekeeping Code Improvements</title>
+
+ <contact>
+ <person>
+ <name>
+ <given>Konstantin</given>
+ <common>Belousov</common>
+ </name>
+ <email>[email protected]</email>
+ </person>
+ </contact>
+
+ <body>
+ <p>Work was done to properly lock the time-keeping code. The
+ existing code correctly interoperated with readers, both kernel-
+ and user-space, giving them lock-less access to the actual data
+ ('timehands') needed to derive the time of day from the
+ timecounter hardware, in the presence of updaters. But updates
+ of the timehands, which are performed by periodic clock
+ interrupts, the ntpd-driven sys_ntp_adjtime(2) syscall, the
+ settimeofday(2) syscall, pps sync, and possibly other sources,
+ were not coordinated. Moreso, the NTP code was locked by Giant,
+ which did not really serve any purpose.</p>
+
+ <p>As result of the work, locking was applied to ensure that any
+ timehands adjustments are performed by single mutator. An
+ interesting case is the parallel modification of the timehands
+ from the top of the kernel, for instance the settimeoday(2)
+ syscall, and a simultaneous clock tick event, where the syscall
+ has already acquired the resources. In this case, it is highly
+ desirable to not block (spin) in the tick handler, and the
+ required adjustments are performed by the already executing call
+ from top half. There, the typical trylock operation is desired,
+ which was surprisingly lacking from our spinlock implementation.
+ mtx_trylock_spin(9) was implemented and is used for this
+ purpose.</p>
+
+ <p>The userspace getttimeofday(2) implementation was enhanced to
+ allow syscall-less operation on machines that use HPET hardwire
+ for timecounters. The HPET algorithm coexists with older
+ RDTSC-based code, allowing dynamic switching of timecounters.
+ HPET registers a page that is mmap(2)-ed readonly by libc into
+ userland application programs' address space as needed.
+ Measurements demonstrated modest improvements in gettimeofday(2)
+ performance, but not unexpectedly even the syscall-less HPET
+ timecounter is slower than invoking a syscall for RDTSC.</p>
+
+ <p>Some not strictly interwined but related code is the
+ time-bound sleep implementation. Handling of races between
+ callouts and the top-half code that sets and processes the
+ timeouts depended on the many fine details of the
+ callout_stop(9) KPI (kernel programming interface). In
+ particular, races or unpunctual KPI changes usually result in
+ the "catch-all" unkillable thread state with the
+ "-" waitchain bug. The sleepqueue timeout code was
+ rewritten to stop depending on the KPI details, which removed
+ the source of recurring bugs, and also surprisingly simplified
+ the code.</p>
+ </body>
+
+ <sponsor>
+ The FreeBSD Foundation
+ </sponsor>
+ </project>
</report>
_______________________________________________
[email protected] mailing list
https://lists.freebsd.org/mailman/listinfo/svn-doc-all
To unsubscribe, send any mail to "[email protected]"