On 2019-09-06 11:15:12 (+0800), Ian Lepore wrote:
On Fri, 2019-09-06 at 01:19 +0000, Philip Paeps wrote:
Author: philip
Date: Fri Sep 6 01:19:31 2019
New Revision: 351918
URL: https://svnweb.freebsd.org/changeset/base/351918
Log:
riscv: default to HZ=100
Most current RISC-V development platforms are not fast enough to
benefit
from the increased granularity provided by HZ=1000.
Sponsored by: Axiado
Modified:
head/sys/kern/subr_param.c
Modified: head/sys/kern/subr_param.c
=====================================================================
=========
--- head/sys/kern/subr_param.c Fri Sep 6 00:06:55 2019 (r351
917)
+++ head/sys/kern/subr_param.c Fri Sep 6 01:19:31 2019 (r351
918)
@@ -61,7 +61,7 @@ __FBSDID("$FreeBSD$");
*/
#ifndef HZ
-# if defined(__mips__) || defined(__arm__)
+# if defined(__mips__) || defined(__arm__) || defined(__riscv)
# define HZ 100
# else
# define HZ 1000
This seems like a bad idea. I've run a 90mhz armv4 chip with HZ=1000
and didn't notice any performance hit from doing so. Almost all arm
kernel config files set HZ as an option, so that define doesn't do
much for arm these days. It probably does still set HZ for various
mips platforms.
I would think 1000 is appropriate for anything modern running at
200mhz or more.
Setting it to 100 has the bad side effect of making things like
msleep(), tsleep(), and pause() (which show up in plenty of drivers)
all have a minimum timeout of 10ms, which is a long long time on
modern hardware.
What benefit do you think you'll get from the lower number?
On systems running at 10s of MHz (or slower, ick), with HZ=1000 you
spend an awful lot of time servicing the timer interrupt and not very
much time doing anything else.
My rationale was that most RISC-V systems (including emulation and FPGA
prototypes) I've encountered are running slower than the tipping point
where HZ=1000 makes sense. With the default of HZ=100, faster
exceptions can still set HZ=1000 in their individual configs.
When the RISC-V world evolves to having more actual silicon and fewer
slow prototypes, I definitely agree this default should be flipped again
for HZ=1000 by default and HZ=100 in the config files for the
exceptions.
Philip
--
Philip Paeps
Senior Reality Engineer
Alternative Enterprises
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