on 25/07/2012 01:10 Jim Harris said the following: > Author: jimharris > Date: Tue Jul 24 22:10:11 2012 > New Revision: 238755 > URL: http://svn.freebsd.org/changeset/base/238755 > > Log: > Add rmb() to tsc_read_##x to enforce serialization of rdtsc captures. > > Intel Architecture Manual specifies that rdtsc instruction is not > serialized, > so without this change, TSC synchronization test would periodically fail, > resulting in use of HPET timecounter instead of TSC-low. This caused > severe performance degradation (40-50%) when running high IO/s workloads > due to > HPET MMIO reads and GEOM stat collection. > > Tests on Xeon E5-2600 (Sandy Bridge) 8C systems were seeing TSC > synchronization > fail approximately 20% of the time.
Should rather the synchronization test be fixed if it's the culprit? Or is this change universally good for the real uses of TSC? > Sponsored by: Intel > Reviewed by: kib > MFC after: 3 days > > Modified: > head/sys/x86/x86/tsc.c > > Modified: head/sys/x86/x86/tsc.c > ============================================================================== > --- head/sys/x86/x86/tsc.c Tue Jul 24 20:15:41 2012 (r238754) > +++ head/sys/x86/x86/tsc.c Tue Jul 24 22:10:11 2012 (r238755) > @@ -328,6 +328,7 @@ init_TSC(void) > > #ifdef SMP > > +/* rmb is required here because rdtsc is not a serializing instruction. */ > #define TSC_READ(x) \ > static void \ > tsc_read_##x(void *arg) \ > @@ -335,6 +336,7 @@ tsc_read_##x(void *arg) \ > uint32_t *tsc = arg; \ > u_int cpu = PCPU_GET(cpuid); \ > \ > + rmb(); \ > tsc[cpu * 3 + x] = rdtsc32(); \ > } > TSC_READ(0) > -- Andriy Gapon _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"
