From owner-freebsd-arch Fri May 26 5: 6:22 2000 Delivered-To: freebsd-arch@freebsd.org Received: from haldjas.folklore.ee (Haldjas.folklore.ee [193.40.6.121]) by hub.freebsd.org (Postfix) with ESMTP id 3FC0A37B548; Fri, 26 May 2000 05:06:14 -0700 (PDT) (envelope-from narvi@haldjas.folklore.ee) Received: from localhost (narvi@localhost) by haldjas.folklore.ee (8.9.3/8.9.3) with SMTP id OAA70460; Fri, 26 May 2000 14:05:49 +0200 (EET) (envelope-from narvi@haldjas.folklore.ee) Date: Fri, 26 May 2000 14:05:48 +0200 (EET) From: Narvi To: Peter Jeremy Cc: Mike Smith , Terry Lambert , arch@FreeBSD.ORG Subject: Re: Preemptive kernel on older X86 hardware In-Reply-To: <00May26.070036est.115225@border.alcanet.com.au> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-arch@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Fri, 26 May 2000, Peter Jeremy wrote: > On Thu, May 25, 2000 at 11:50:12AM +1000, Terry Lambert wrote: > >What would be the "NOP" overhead on a pipelined instruction cache? > > On a 486, a NOP takes 1 cycle to execute. I suspect it's zero on the > more recent processors. As Terry points out, there's also the fetch > cost - but I-fetches are (effectively) free unless the processor bus > is saturated. > NOP cost is not free on a lot of processors. The OOO processors have more resources than just 'execute' that are contended for. At the very least, it eats up issue slots. When and how they are removed from the instruction stream is a processor specific issue and varies widely. > > I don't think this'd work on > >i386, since the replacement code sequence is larger... > > You just have the larger code as the default and patch the smaller > sequence if appropriate. > > Peter > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-arch" in the body of the message