Date: Mon, 11 Jun 2012 10:22:31 +0100 From: "Robert N. M. Watson" <rwatson@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: arch@freebsd.org Subject: Re: Fast gettimeofday(2) and clock_gettime(2) Message-ID: <E316FFF8-7718-45C8-88F7-3A725B54E976@freebsd.org> In-Reply-To: <20120611091811.GA2337@deviant.kiev.zoral.com.ua> References: <20120606165115.GQ85127@deviant.kiev.zoral.com.ua> <alpine.BSF.2.00.1206110952570.78881@fledge.watson.org> <20120611091811.GA2337@deviant.kiev.zoral.com.ua>
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On 11 Jun 2012, at 10:18, Konstantin Belousov wrote: >> 3) For the purposes of adaptive mutexes in userspace, it really would be >> quite >> nice to know whether remote threads are running or not, in the same way >> that cheap access to remote thread run state in the kernel makes for much >> more efficient adaptive spinning. I wonder if we could use this >> mechanism >> for that purpose as well. I guess for now, at least, you're using a >> single >> global page, but in the future, per-process pages might be quite >> beneficial. > > The per-process page looks almost undoable. I think that what could be > made working, although with some hacks, is per-CPU page, and the page > content update on context switch. This could benefit trivial system calls > like getpid(), getppid() and others, but obviously cause increased context > switch latency. > > Per-CPU page would then solve the proposal of having an indicator of > other threads running. I am not sure how much do we care of the potential > information leak there. FYI, the FreeBSD/MIPS kernel already makes use of an MD per-thread page using a reserved TLB entry switched on each kernel context switch. Interestingly, this model effectively conflicts (semantically) with the higher-level MI per-CPU mechanism. It would be nice to unify across the layers within the kernel, even if not all the way to userspace. Roberthelp
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