From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: SMP kernels on single processor machines From: Benjamin Herrenschmidt To: Giuliano Pochini Cc: linuxppc-dev list , Lee Braiden In-Reply-To: References: Content-Type: text/plain Message-Id: <1085137865.6133.2.camel@gaston> Mime-Version: 1.0 Date: Fri, 21 May 2004 21:11:06 +1000 Sender: owner-linuxppc-dev@lists.linuxppc.org List-Id: On Fri, 2004-05-21 at 20:34, Giuliano Pochini wrote: > > 2.6.6 has the CONFIG_PREEMPT option so I thought it was stable. > Isn't it ? What are the known problem ? Is only preempt+smp > known to have problems ? Well, there are 3 different things. One is stability. I don't know of any ppc-specific problem with preempt, though I do have reports of people experiencing problems (mostly various kinds of segfaults) when it's enabled. I'm not sure what is to blame at this point, possibly one of the filesystems. Another is overhead. Preempt definitely adds overhead to the kernel. The spinlocks, on an UP kernel, are mostly NOPs, while with preempt, they are actually implemented (among others). Overall, preempt adds overhead to the kernel. Finally, the supposed benefit. Mostly a myth imho. For those who didn't get it (yes, that happens), the kernel, even without preempt, will preempt user processes :) Linux has always been a preemptible operating system. CONFIG_PREEMPT only concerns the ability for the kernel to preempt itself when a process triggers a potentially long operations within the kernel. Ben. ** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/