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>
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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
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