Date: Fri, 28 Oct 2005 15:20:46 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Robert Watson <rwatson@FreeBSD.org> Cc: Pertti Kosunen <pertti.kosunen@pp.nic.fi>, David Xu <davidxu@freebsd.org>, "Yuriy N. Shkandybin" <jura@networks.ru>, current@freebsd.org Subject: Re: Timers and timing, was: MySQL Performance 6.0rc1 Message-ID: <32412.1130505646@critter.freebsd.dk> In-Reply-To: Your message of "Fri, 28 Oct 2005 14:08:01 BST." <20051028140556.W20147@fledge.watson.org>
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In message <20051028140556.W20147@fledge.watson.org>, Robert Watson writes: > >On Fri, 28 Oct 2005, David Xu wrote: > >> Poul-Henning Kamp wrote: >>> In message <4361FDBE.7000500@freebsd.org>, David Xu writes: >>> >>> the correct way to optimize this would be to add a time(2) systemcall >>> which returns the value of the kernel global time_second. >> >> Can we make a page in kernel address space which is readable my user >> code? put the variable in the page, I know read an integer is atomic-op, >> needn't lock, so syscall is not needed. > >This approach has a lot of merit, as we can also potentially export other >information there (such as kernel preferences for system call mechanisms). Yes, there are many advantages to this approach, but we need a solution to the API versioning problem before we head that way. For anyone wanting to look at this, three are a number of nasties to remember: 1. How does userland get hold of the page ? Does it open a magic device ? Use a magic syscall ? Or does all processes just get the page by default ? 2. Where in the address space do we put it ? 3. Layout and alignment issues. Remember that things change size over time. (Version numbers for each element ?) And that cross- arch support is desirable (32bit i386 binaries on 64bit amd64 arch) 4. Do guarantee a syscall fallback for all facilities if there is version skew, or do we abort the program ? 5. Do we want a global system page and a per process page while we are at it. There is plenty of stuff we could put in the per-proc page: pid, ppid, resource usage, proctitle etc. >On the other hand, a lower risk change might be to simply add a new CLOCK_ >type for lower resolution, and have a timer synchronize a variable to the >system clock once every 1/10 of a second. This avoids having to muck with >VM layout, etc. Is the CLOCK_* namespace ours to muck about with in the first place ? -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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