Skip site navigation (1)Skip section navigation (2)
Date:      Wed, 30 May 2001 20:37:13 -0400 (EDT)
From:      Andrew Gallatin <gallatin@cs.duke.edu>
To:        Peter Jeremy <peter.jeremy@alcatel.com.au>
Cc:        freebsd-alpha@FreeBSD.ORG
Subject:   Re: Abyssmal interrupt latency with -current
Message-ID:  <15125.37433.101479.231284@grasshopper.cs.duke.edu>
In-Reply-To: <20010531093454.X89950@gsmx07.alcatel.com.au>
References:  <20010531093454.X89950@gsmx07.alcatel.com.au>

next in thread | previous in thread | raw e-mail | index | archive | help

Peter Jeremy writes:
 > 
 > Once control actually gets to siointr, grabbing the sio mutex takes
 > another 2.7usec (median) - again quite high.

Perhaps this is so abysmally slow due to critical_enter/exit.  They
manipulate the IPL and each make a call into palcode, so they are
_very_ expensive. You should time just those functions and see.  In
fact, you really should un-inline them and give iprobe a try...

<...>

 > Does anyone have any ideas what could be leading to such ludicrous
 > interrupt latencies?

Pal + SMPng + painfully slow hardware == ludicrous interrupt latencies.

To be fair, its not all SMPng's fault.  Alpha interrupt latencies have
always left quite a bit to be desired.  Pal adds an extra layer of
abstraction and slowness between the actual hardware and the OS.
There's no way an alpha can be as tight as a platform where the OS has
access to the actual hardware.

I suspect the multia's osf/1 palcode may be considerably worse than
the palcode on most platforms.  Consider that the Multia was never
designed to run OSF/1; it was supposed to be a glorified Xterm running
alpha NT.  I seriously doubt that DEC spent much time optimizing the
palcode on a low-margin, loss-leading piece of junk that was dog-slow
and essentially obsolete the day it was introduced.

I should really resurrect my Pamette and actually compare device
interrupt latencies for Tru64, -stable and -current on one of our
miatas before they get recycled to other departments.

Drew




To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-alpha" in the body of the message




Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?15125.37433.101479.231284>