From nobody Sat Oct 9 21:21:39 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 CF1E817EEFE1 for ; Sat, 9 Oct 2021 21:21:49 +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 4HRdKn3fRyz4kgT for ; Sat, 9 Oct 2021 21:21:49 +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 199LLe9w033850 (version=TLSv1.3 cipher=TLS_AES_256_GCM_SHA384 bits=256 verify=NO); Sun, 10 Oct 2021 00:21:43 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua 199LLe9w033850 Received: (from kostik@localhost) by tom.home (8.16.1/8.16.1/Submit) id 199LLdxZ033849; Sun, 10 Oct 2021 00:21:39 +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 00:21:39 +0300 From: Konstantin Belousov To: Poul-Henning Kamp 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> <202110092107.199L7T4j059128@critter.freebsd.dk> 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: <202110092107.199L7T4j059128@critter.freebsd.dk> 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: 4HRdKn3fRyz4kgT X-Spamd-Bar: ---- Authentication-Results: mx1.freebsd.org; none X-Spamd-Result: default: False [-4.00 / 15.00]; REPLY(-4.00)[] X-ThisMailContainsUnwantedMimeParts: N On Sat, Oct 09, 2021 at 09:07:29PM +0000, Poul-Henning Kamp wrote: > -------- > Warner Losh writes: > > > > 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? > > The reason there were originally 10 timehands was that latency in > the early SMP kernels was ... ehh ... variable ... and some of the > time-counters rolled over quite fast compared to that. > > I really hope no relevant current hardware has that problem. The current algorithm to read timehands is resilient to the wrap-out of the current hand. You really need to experience enourmous delays in the reader loop to make it lock-step with tc_windup() updates, in which case it could indeed be better to have more than two timehands. I believe it was Ian who reported that 16 timehands worked better for him than 2.