Date: Thu, 7 Jun 2012 20:28:39 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-arch@freebsd.org Subject: Re: Fast gettimeofday(2) and clock_gettime(2) Message-ID: <20120607172839.GZ85127@deviant.kiev.zoral.com.ua> In-Reply-To: <201206070850.55751.jhb@freebsd.org> References: <20120606165115.GQ85127@deviant.kiev.zoral.com.ua> <201206061423.53179.jhb@freebsd.org> <20120606205938.GS85127@deviant.kiev.zoral.com.ua> <201206070850.55751.jhb@freebsd.org>
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--YBGzgpgHAney5ErF Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Thu, Jun 07, 2012 at 08:50:55AM -0400, John Baldwin wrote: > On Wednesday, June 06, 2012 4:59:38 pm Konstantin Belousov wrote: > > On Wed, Jun 06, 2012 at 02:23:53PM -0400, 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. > > >=20 > > > In general this looks good but I see a few nits / races: > > >=20 > > > 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 changed the code to be much more similar to the kern_tc.c. I (re)added > > the generation field, which is set to 0 upon kernel touching timehands. >=20 > Thank you. BTW, I think we should use atomic_load_acq_int() on both acce= sses=20 > to th_gen (and the in-kernel binuptime should do the same). I realize th= is > requires using rmb before the while condition in userland since we can't > use atomic_load_acq_int() here. I think it should also use=20 > atomic_store_rel_int() for both stores to th_gen during the tc_windup() > callback. This is done. On the other hand, I removed a store_rel from updating tk_current, since it is after enabling store to th_gen, and the order there does not matter. I also did some restructuring of the userspace, removing layers that Bruce did not liked. Now top-level functions directly call binuptime(). I also shortened the preliminary operations by caching timekeep pointer. Its double-initialization is safe. Latest version is at http://people.freebsd.org/~kib/misc/moronix.4.patch I probably move all shared page helpers to separate file from kern_exec.c, but this will happen after moronix is committed. --YBGzgpgHAney5ErF Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk/Q5McACgkQC3+MBN1Mb4goxQCg1CEB9/qDJ7WNNVdNleSpqiUS kZwAniRrYMNQOjHycMeeoCOu4ixtChdl =j52Z -----END PGP SIGNATURE----- --YBGzgpgHAney5ErF--
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