Date: Thu, 11 Jul 2002 12:30:33 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: "Ken Menzel" <kenm@icarz.com>, "Hartmann, O." <ohartman@klima.physik.uni-mainz.de> Cc: <freebsd-stable@FreeBSD.ORG> Subject: Re: tuning(7) request was: Re: Performance boost with kernel options in FBSD 4.6 Message-ID: <200207111930.g6BJUX5m096974@apollo.backplane.com> References: <20020710104730.L10343-100000@klima.physik.uni-mainz.de> <04a601c228dc$c6dbb980$681663cf@icarz.com>
next in thread | previous in thread | raw e-mail | index | archive | help
:
:Hi,
:If it's possible this makes a difference can we get a note about HZ
:added to the tuning(7) man page?
:
:Thanks Ken
I could put a general admonition in tuning(7) about Hz, but the
performance effects are going to be highly dependant on the situation.
Generally speaking aggregate performance will not improve if you increase
Hz, but I can see how perceived performance might improve in
certain specific situations such as having a lot of X clients talking
to the server at the same time.
The issue with X clients is that a single interactive operation done on
the client may result in dozens of interactive packet ops occuring
between client and server, many of which cannot be pipelined. In this
situation the priority scheduling mechanism tends to break down because
the server processes are utilizing a huge amount of cpu but are still
classified as being interactive due to short term I/O waits. Several
clients may monopolize the server in this fashion and cause obvious
lag for the remaining clients. For example, if a couple of clients
run 'xengine' the other clients could suffer greatly.
An increased switching rate (increasing HZ) may be useful in the above
situation. Still, I would not recommend increasing Hz above 500 (2ms).
10000 (100uS) is just plain insane.
I think it is high time that we changed the system default on 'fast'
machines (anything over 300 MHz) from 100 to 250. 100 is archaic.
We will not see detrimental cache side effects until we get above 1000
or so (my guess) so I think '250' as a default instead of 100 is a
good idea.
But for most people it just doesn't matter.
-Matt
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-stable" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200207111930.g6BJUX5m096974>