# git: dee01da58a27 - stable/13 - Correctly measure system load averages > 1024

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**Alan Somers <asomers_at_FreeBSD.org>**

*From:***Mon, 23 May 2022 19:11:51 UTC**

*Date:*The branch stable/13 has been updated by asomers: URL: https://cgit.FreeBSD.org/src/commit/?id=dee01da58a275b1cdc21b6211b26223968431449 commit dee01da58a275b1cdc21b6211b26223968431449 Author: Alan Somers <asomers@FreeBSD.org> AuthorDate: 2022-05-05 21:35:23 +0000 Commit: Alan Somers <asomers@FreeBSD.org> CommitDate: 2022-05-23 19:11:23 +0000 Correctly measure system load averages > 1024 The old fixed-point arithmetic used for calculating load averages had an overflow at 1024. So on systems with extremely high load, the observed load average would actually fall back to 0 and shoot up again, creating a kind of sawtooth graph. Fix this by using 64-bit math internally, while still reporting the load average to userspace as a 32-bit number. Sponsored by: Axcient Reviewed by: imp Differential Revision: https://reviews.freebsd.org/D35134 (cherry picked from commit 1d2421ad8b6d508ef155752bdfc5948f7373bac3) --- sys/kern/kern_synch.c | 9 +++++---- sys/kern/tty_info.c | 2 +- sys/sys/param.h | 8 ++++---- 3 files changed, 10 insertions(+), 9 deletions(-) diff --git a/sys/kern/kern_synch.c b/sys/kern/kern_synch.c index 88f47ba78601..5abc38d64296 100644 --- a/sys/kern/kern_synch.c +++ b/sys/kern/kern_synch.c @@ -87,7 +87,7 @@ struct loadavg averunnable = * Constants for averages over 1, 5, and 15 minutes * when sampling at 5 second intervals. */ -static fixpt_t cexp[3] = { +static uint64_t cexp[3] = { 0.9200444146293232 * FSCALE, /* exp(-1/12) */ 0.9834714538216174 * FSCALE, /* exp(-1/60) */ 0.9944598480048967 * FSCALE, /* exp(-1/180) */ @@ -610,14 +610,15 @@ setrunnable(struct thread *td, int srqflags) static void loadav(void *arg) { - int i, nrun; + int i; + uint64_t nrun; struct loadavg *avg; - nrun = sched_load(); + nrun = (uint64_t)sched_load(); avg = &averunnable; for (i = 0; i < 3; i++) - avg->ldavg[i] = (cexp[i] * avg->ldavg[i] + + avg->ldavg[i] = (cexp[i] * (uint64_t)avg->ldavg[i] + nrun * FSCALE * (FSCALE - cexp[i])) >> FSHIFT; /* diff --git a/sys/kern/tty_info.c b/sys/kern/tty_info.c index 60675557e4ed..237aa47a18da 100644 --- a/sys/kern/tty_info.c +++ b/sys/kern/tty_info.c @@ -302,7 +302,7 @@ tty_info(struct tty *tp) sbuf_set_drain(&sb, sbuf_tty_drain, tp); /* Print load average. */ - load = (averunnable.ldavg[0] * 100 + FSCALE / 2) >> FSHIFT; + load = ((int64_t)averunnable.ldavg[0] * 100 + FSCALE / 2) >> FSHIFT; sbuf_printf(&sb, "%sload: %d.%02d ", tp->t_column == 0 ? "" : "\n", load / 100, load % 100); diff --git a/sys/sys/param.h b/sys/sys/param.h index a0f1b9f7945b..24011244449e 100644 --- a/sys/sys/param.h +++ b/sys/sys/param.h @@ -343,12 +343,12 @@ __END_DECLS * Scale factor for scaled integers used to count %cpu time and load avgs. * * The number of CPU `tick's that map to a unique `%age' can be expressed - * by the formula (1 / (2 ^ (FSHIFT - 11))). The maximum load average that - * can be calculated (assuming 32 bits) can be closely approximated using - * the formula (2 ^ (2 * (16 - FSHIFT))) for (FSHIFT < 15). + * by the formula (1 / (2 ^ (FSHIFT - 11))). Since the intermediate + * calculation is done with 64-bit precision, the maximum load average that can + * be calculated is approximately 2^32 / FSCALE. * * For the scheduler to maintain a 1:1 mapping of CPU `tick' to `%age', - * FSHIFT must be at least 11; this gives us a maximum load avg of ~1024. + * FSHIFT must be at least 11. This gives a maximum load avg of 2 million. */ #define FSHIFT 11 /* bits to right of fixed binary point */ #define FSCALE (1<<FSHIFT)