On Tuesday 02 October 2007 09:49:34 am Bruce Evans wrote:
> Apparently, there is a scaling bug for hz or extra interrupts for
> the larger hz help, and the default preempt_thresh is not best.
-current on i386 and amd64 does a very poor job of scaling stathz and profhz
with hz, so this may explain problems at hz=100. I have an attempt to make
stathz and profhz more sane while also trying to not always drive the lapic
timer at hz * 2. I used to have a test program that would display all the
frequencies for different 'hz' values but have misplaced it. :(
--- //depot/vendor/freebsd/src/sys/amd64/amd64/local_apic.c 2007/09/11
22:57:37
+++ //depot/user/jhb/acpipci/amd64/amd64/local_apic.c 2007/09/24 17:24:13
@@ -72,8 +72,6 @@
CTASSERT(IPI_STOP < APIC_SPURIOUS_INT);
#define LAPIC_TIMER_HZ_DIVIDER 2
-#define LAPIC_TIMER_STATHZ_DIVIDER 15
-#define LAPIC_TIMER_PROFHZ_DIVIDER 3
/* Magic IRQ values for the timer and syscalls. */
#define IRQ_TIMER (NUM_IO_INTS + 1)
@@ -383,13 +381,24 @@
lapic_timer_divisor, value);
/*
- * We will drive the timer at a small multiple of hz and drive
- * both of the other timers with similarly small but relatively
- * prime divisors.
+ * We want to run stathz in the neighborhood of 128hz. We would
+ * like profhz to run as often as possible, so we let it run on
+ * each clock tick. We try to honor the requested 'hz' value as
+ * much as possible.
+ *
+ * If 'hz' is above 1500, then we just let the lapic timer
+ * (and profhz) run at hz. If 'hz' is below 1500 but above
+ * 750, then we let the lapic timer run at 2 * 'hz'. If 'hz'
+ * is below 750 then we let the lapic timer run at 4 * 'hz'.
*/
- lapic_timer_hz = hz * LAPIC_TIMER_HZ_DIVIDER;
- stathz = lapic_timer_hz / LAPIC_TIMER_STATHZ_DIVIDER;
- profhz = lapic_timer_hz / LAPIC_TIMER_PROFHZ_DIVIDER;
+ if (hz >= 1500)
+ lapic_timer_hz = hz;
+ else if (hz >= 750)
+ lapic_timer_hz = hz * 2;
+ else
+ lapic_timer_hz = hz * 4;
+ stathz = lapic_timer_hz / (lapic_timer_hz * 128);
+ profhz = lapic_timer_hz;
lapic_timer_period = value / lapic_timer_hz;
/*
--- //depot/vendor/freebsd/src/sys/i386/i386/local_apic.c 2007/09/11
22:57:37
+++ //depot/user/jhb/acpipci/i386/i386/local_apic.c 2007/09/24 17:24:13
@@ -72,8 +72,6 @@
CTASSERT(IPI_STOP < APIC_SPURIOUS_INT);
#define LAPIC_TIMER_HZ_DIVIDER 2
-#define LAPIC_TIMER_STATHZ_DIVIDER 15
-#define LAPIC_TIMER_PROFHZ_DIVIDER 3
/* Magic IRQ values for the timer and syscalls. */
#define IRQ_TIMER (NUM_IO_INTS + 1)
@@ -385,13 +383,24 @@
lapic_timer_divisor, value);
/*
- * We will drive the timer at a small multiple of hz and drive
- * both of the other timers with similarly small but relatively
- * prime divisors.
+ * We want to run stathz in the neighborhood of 128hz. We would
+ * like profhz to run as often as possible, so we let it run on
+ * each clock tick. We try to honor the requested 'hz' value as
+ * much as possible.
+ *
+ * If 'hz' is above 1500, then we just let the lapic timer
+ * (and profhz) run at hz. If 'hz' is below 1500 but above
+ * 750, then we let the lapic timer run at 2 * 'hz'. If 'hz'
+ * is below 750 then we let the lapic timer run at 4 * 'hz'.
*/
- lapic_timer_hz = hz * LAPIC_TIMER_HZ_DIVIDER;
- stathz = lapic_timer_hz / LAPIC_TIMER_STATHZ_DIVIDER;
- profhz = lapic_timer_hz / LAPIC_TIMER_PROFHZ_DIVIDER;
+ if (hz >= 1500)
+ lapic_timer_hz = hz;
+ else if (hz >= 750)
+ lapic_timer_hz = hz * 2;
+ else
+ lapic_timer_hz = hz * 4;
+ stathz = lapic_timer_hz / (lapic_timer_hz * 128);
+ profhz = lapic_timer_hz;
lapic_timer_period = value / lapic_timer_hz;
/*
--
John Baldwin
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