git: 0f882bb14a0e - main - NOTES: Update the HZ entry with latest advice.
Warner Losh
imp at FreeBSD.org
Fri Jun 18 14:43:49 UTC 2021
The branch main has been updated by imp:
URL: https://cgit.FreeBSD.org/src/commit/?id=0f882bb14a0e772bf901b1a3e40d028a4165fbd9
commit 0f882bb14a0e772bf901b1a3e40d028a4165fbd9
Author: Warner Losh <imp at FreeBSD.org>
AuthorDate: 2021-06-18 14:39:18 +0000
Commit: Warner Losh <imp at FreeBSD.org>
CommitDate: 2021-06-18 14:41:56 +0000
NOTES: Update the HZ entry with latest advice.
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30804
---
sys/conf/NOTES | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)
diff --git a/sys/conf/NOTES b/sys/conf/NOTES
index 40063c09382f..89dd572986bc 100644
--- a/sys/conf/NOTES
+++ b/sys/conf/NOTES
@@ -1047,8 +1047,7 @@ options TCP_SIGNATURE #include support for RFC 2385
# DUMMYNET enables the "dummynet" bandwidth limiter. You need IPFIREWALL
# as well. See dummynet(4) and ipfw(8) for more info. When you run
-# DUMMYNET it is advisable to also have at least "options HZ=1000" to achieve
-# a smooth scheduling of the traffic.
+# DUMMYNET, HZ/kern.hz should be at least 1000 for adequate response.
options DUMMYNET
# The DEBUGNET option enables a basic debug/panic-time networking API. It
@@ -1237,14 +1236,11 @@ options CAPABILITY_MODE # sandboxes with no global namespace access
#####################################################################
# CLOCK OPTIONS
-# The granularity of operation is controlled by the kernel option HZ whose
-# default value (1000 on most architectures) means a granularity of 1ms
-# (1s/HZ). Historically, the default was 100, but finer granularity is
-# required for DUMMYNET and other systems on modern hardware. There are
-# reasonable arguments that HZ should, in fact, be 100 still; consider,
-# that reducing the granularity too much might cause excessive overhead in
-# clock interrupt processing, potentially causing ticks to be missed and thus
-# actually reducing the accuracy of operation.
+# The granularity of operation is controlled by the kernel option HZ (default
+# 1000 or 1ms). Virtual machines guests default to an HZ value of 100. Lower
+# values produce less overhead at the expense of reduce accuracy in delivery of
+# kernel timeouts. With the adaptive tick code, lower values produce less
+# benefit than in the past.
options HZ=100
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