From nobody Sun Oct 10 17:39:17 2021 X-Original-To: freebsd-hackers@mlmmj.nyi.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1]) by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id A623217E348C for ; Sun, 10 Oct 2021 17:39:35 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 4HS8Lv2r7cz3QcG; Sun, 10 Oct 2021 17:39:35 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kib@localhost [127.0.0.1]) by kib.kiev.ua (8.16.1/8.16.1) with ESMTPS id 19AHdHoK053463 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Sun, 10 Oct 2021 20:39:21 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua 19AHdHoK053463 Received: (from kostik@localhost) by tom.home (8.16.1/8.16.1/Submit) id 19AHdHMm053462; Sun, 10 Oct 2021 20:39:17 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Sun, 10 Oct 2021 20:39:17 +0300 From: Konstantin Belousov To: Ian Lepore Cc: Warner Losh , Sebastian Huber , FreeBSD Hackers Subject: Re: Why was the timehands_count sysctl added? Message-ID: References: <2d1d2a6d-ec6b-7f52-8af3-09a833c52820@embedded-brains.de> List-Id: Technical discussions relating to FreeBSD List-Archive: https://lists.freebsd.org/archives/freebsd-hackers List-Help: List-Post: List-Subscribe: List-Unsubscribe: Sender: owner-freebsd-hackers@freebsd.org MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FORGED_GMAIL_RCVD,FREEMAIL_FROM, NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.5 X-Spam-Checker-Version: SpamAssassin 3.4.5 (2021-03-20) on tom.home X-Rspamd-Queue-Id: 4HS8Lv2r7cz3QcG X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] X-Spam: Yes X-ThisMailContainsUnwantedMimeParts: N On Sun, Oct 10, 2021 at 11:15:39AM -0600, Ian Lepore wrote: > On Sat, 2021-10-09 at 14:54 -0600, Warner Losh wrote: > > On Sat, Oct 9, 2021, 1:56 PM Konstantin Belousov > > > > wrote: > > > > > On Sat, Oct 09, 2021 at 09:41:28PM +0200, Sebastian Huber wrote: > > > > Hello, > > > > > > > > I synchronize currently the port of the FreeBSD timecounters to > > > > RTEMS and > > > > came across a change in 2019 which I do not understand. Some time > > > > ago the > > > > timehands were reduced from 10 to two: > > > > > > > > https://reviews.freebsd.org/D7302 > > > > > > > > In 2019 this changed again to be up to 16: > > > > > > > > https://reviews.freebsd.org/D21563 > > > This review did not changed it back to 16, the default value is > > > still 2. > > > It allows to bump the number of used timehands, but normally > > > systems run > > > with only 2. > > > > > > > > > > > The corresponding sysctl is not documented: > > > > > > > > https://www.freebsd.org/cgi/man.cgi?query=timecounters&sektion=4 > > > > > > > > Does someone know why this sysctl to select the count of > > > > timehands was > > > > added? Is this a performance optimization for some systems? > > > > > > To allow for experimentation, and to satisfy some requests where > > > people > > > wanted to have more that 2 timehands. > > > > > > > When would someone want that? What's the use case? > > > > Warner > > > > > > > One known usecase is a single-cpu (non-SMP) system that uses a PPS > signal. With only two timehands, a pps pulse that arrives while the > system is in an idle-sleep (wait-for-interrupt) state will switch > timehands too many times between the wakeup and the capture and trying > to use the capture data, so that the th generation count never matches, > and in effect you never capture a PPS pulse. I found that on a > BeagleBone system. It takes a minimum of 3 timehands for it to work > right. So should this system automatically adjust the number of timehands? There were no similar complaints on UP i386, at least I did not see them.