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* [newlib-cygwin] Correctly measure system load averages > 1024
@ 2022-07-11 11:55 Sebastian Huber
0 siblings, 0 replies; only message in thread
From: Sebastian Huber @ 2022-07-11 11:55 UTC (permalink / raw)
Author: Alan Somers <asomers@FreeBSD.org>
Date: Thu May 5 15:35:23 2022 -0600
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
newlib/libc/sys/rtems/include/sys/param.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/newlib/libc/sys/rtems/include/sys/param.h b/newlib/libc/sys/rtems/include/sys/param.h
index c346453bf..9596ecf52 100644
@@ -248,12 +248,12 @@
* 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)
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2022-07-11 11:55 [newlib-cygwin] Correctly measure system load averages > 1024 Sebastian Huber
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