Date: Thu, 7 Jun 2012 08:10:08 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-arch@freebsd.org Cc: Konstantin Belousov <kostikbel@gmail.com> Subject: Re: Fast gettimeofday(2) and clock_gettime(2) Message-ID: <201206070810.08166.jhb@freebsd.org> In-Reply-To: <20120607084229.C1474@besplex.bde.org> References: <20120606165115.GQ85127@deviant.kiev.zoral.com.ua> <201206061423.53179.jhb@freebsd.org> <20120607084229.C1474@besplex.bde.org>
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On Wednesday, June 06, 2012 9:35:49 pm Bruce Evans wrote: > On Wed, 6 Jun 2012, John Baldwin wrote: > > > On Wednesday, June 06, 2012 12:51:15 pm Konstantin Belousov wrote: > >> A positive result from the recent flame-bait on arch@ is the working > >> implementation of the fast gettimeofday(2) and clock_gettime(2). The > >> speedup I see is around 6-7x on the 2600K. I think the speedup could > >> be even bigger on the previous generation of CPUs, where lock > >> operations and syscall entry are costlier. A sample test runs of > >> tools/tools/syscall_timing are presented at the end of message. > > > > In general this looks good but I see a few nits / races: > > It is awefully (sic) complete and large. The patch is almost twice as > large as the entire kern_tc.c in FreeBSD-4, and that was quite bloated. > > > 1) You don't follow the model of clearing tk_current to 0 while you > > are updating the structure that the in-kernel timecounter code > > uses. This also means you have to avoid using a tk_current of 0 > > and that userland has to keep spinning as long as tk_current is 0. > > Without this I believe userland can read a partially updated > > structure. > > I thought that too at first, but after looking at the patch decided > that it may be correct, but is too hard for me to understand. > Urk, we both missed that tk_current is an index into the timehands > array, so it cannot act as a generation count and it seems to be harder > to lock. Ugh, so it goes a long way to emulate the timehands array in userland. As I mentioned previously, I consider the timehands array to be a bug. However, I do think the generation count in the in-kernel timehands structure is useful and should be kept (and follow the same model of setting it to 0 before doing updates, then updating the structure, then setting the new generation). -- John Baldwin
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