Date: Wed, 2 Oct 2019 08:48:33 +0200 From: Sebastian Huber <sebastian.huber@embedded-brains.de> To: Konstantin Belousov <kostikbel@gmail.com> Cc: FreeBSD <freebsd-hackers@freebsd.org> Subject: Re: Make timehands count selectable at boottime? Message-ID: <c705f37c-a7d5-e656-f8af-e91de7f743d5@embedded-brains.de> In-Reply-To: <20191001143718.GU44691@kib.kiev.ua> References: <0d534569-504f-52c2-f09c-b977fe80843d@embedded-brains.de> <20191001143718.GU44691@kib.kiev.ua>
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On 01/10/2019 16:37, Konstantin Belousov wrote: > On Tue, Oct 01, 2019 at 12:48:46PM +0200, Sebastian Huber wrote: >> Hello, >> >> a recent commit made the timehands count selectable at boottime: >> >> https://reviews.freebsd.org/D21563 >> >> What was the reason for this change? >=20 > See the followup thread on the svn-src@ list for this revision. Thanks for the hint. So, this change is related to the PPS handling and=20 the things which happen between a pps_capture() and pps_event(). What I don't understand is why more timehands are a problem for the=20 consumer side. For example: void binuptime(struct bintime *bt) { struct timehands *th; u_int gen; do { th =3D timehands; gen =3D atomic_load_acq_int(&th->th_generation); *bt =3D th->th_offset; bintime_addx(bt, th->th_scale * tc_delta(th)); atomic_thread_fence_acq(); } while (gen =3D=3D 0 || gen !=3D th->th_generation); } With more timehands the time frame in which th->th_generation remains=20 constant is longer the more timehands you have. So, it is less likely=20 that you have to execute the loop body twice. The count of timehands is=20 limited by the timecounter frequency and mask, and the hz configuration. --=20 Sebastian Huber, embedded brains GmbH Address : Dornierstr. 4, D-82178 Puchheim, Germany Phone : +49 89 189 47 41-16 Fax : +49 89 189 47 41-09 E-Mail : sebastian.huber@embedded-brains.de PGP : Public key available on request. Diese Nachricht ist keine gesch=C3=A4ftliche Mitteilung im Sinne des EHUG= .
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