From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD68EC43217 for ; Tue, 8 Nov 2022 17:28:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234282AbiKHR2o (ORCPT ); Tue, 8 Nov 2022 12:28:44 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32850 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233891AbiKHR2l (ORCPT ); Tue, 8 Nov 2022 12:28:41 -0500 Received: from mail-io1-xd29.google.com (mail-io1-xd29.google.com [IPv6:2607:f8b0:4864:20::d29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1FCFA183AC for ; Tue, 8 Nov 2022 09:28:40 -0800 (PST) Received: by mail-io1-xd29.google.com with SMTP id e189so12017439iof.1 for ; Tue, 08 Nov 2022 09:28:40 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20210112.gappssmtp.com; s=20210112; h=content-transfer-encoding:in-reply-to:from:references:cc:to :content-language:subject:user-agent:mime-version:date:message-id :from:to:cc:subject:date:message-id:reply-to; bh=Z2BqUuITXKXD3x974ZU/jMzss9BnT6lrNhOqUMpYtPU=; b=3W+pnKFRVAc9AQTDhO6El3k7416sI0KF0+A0KcOgZqtz4zChJ0mWlwvvkjDgcRCXGs 7q13hRRgz7VtL6M4f3qd8jicuCqaZ9KK+xfY7BWsD9ti0r/4yDkGECrGZe2VTX1jcAp3 Y6jq8Ia5az44x8Gh+Uo4V5zifQecoh5zpoJeSWaztPFsxvsCyJd7Mvep83HnCBmNgVy0 v1QVdfz+2lqsPS2Km3b6+EswLM7X98619OrHEjzxEb0JBedzncAxYf+4kIyqxaBtY0c8 7dAlJkPTNug6ChKA/MBWDri1mmANV4GegKltjRmX987KpX1z4HC1izXPOS4PtoijwHEz NxhA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:in-reply-to:from:references:cc:to :content-language:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Z2BqUuITXKXD3x974ZU/jMzss9BnT6lrNhOqUMpYtPU=; b=aIk/pZ3evfRYonnQvVk6/yiR1WaBksPNWE/N7hjuOz3/RY7kqZT7qDfpCxds8qOXH2 e7lLd/qbr8R3auEn1Rqd3LpAyfENtKMBxLd+Owr/zH8LiNah24+ePtoAbkdjDHf5OsWw rN9CFM0b0YfHz9UaL0istFq6Vpg0AolDA/3stW36ZlPndk3aO8CFdMRv3D5LrmLQBVrl cciojkfkizzzRtukBs2g9wwIDE/9gfzKWivcmmOkrt4CdAKIV7NlG7cgjFHHsoPQ1BJh LwxlteiHXSDI6lDcAh9lDaVakN/EUvkP8CkXyiOkrkY5mJBT/KeBxE1DVGmaLXWrQmVL HVtQ== X-Gm-Message-State: ACrzQf0C717zoh7IcsLSInHeWb6tlZwnd+o3Ice9hmk5gbxAVc3dv3C6 1L5Ve8nDzkfCHdY8AWc6ubLXzw== X-Google-Smtp-Source: AMsMyM7qVs172OBX9dAWncPdKSGK0cORI7wWqLrF2XSDZUWhply6enufPx9b1ilUqvWuH6aTHnX7Fw== X-Received: by 2002:a5d:9659:0:b0:6bb:8ed4:ac6c with SMTP id d25-20020a5d9659000000b006bb8ed4ac6cmr885964ios.6.1667928519364; Tue, 08 Nov 2022 09:28:39 -0800 (PST) Received: from [192.168.1.94] ([207.135.234.126]) by smtp.gmail.com with ESMTPSA id g5-20020a028505000000b0034294118e1bsm3994086jai.126.2022.11.08.09.28.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 08 Nov 2022 09:28:38 -0800 (PST) Message-ID: Date: Tue, 8 Nov 2022 10:28:37 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101 Thunderbird/102.4.1 Subject: Re: [PATCHSET v3 0/5] Add support for epoll min_wait Content-Language: en-US To: Stefan Hajnoczi Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org References: <4281b354-d67d-2883-d966-a7816ed4f811@kernel.dk> <93fa2da5-c81a-d7f8-115c-511ed14dcdbb@kernel.dk> <75c8f5fe-6d5f-32a9-1417-818246126789@kernel.dk> From: Jens Axboe In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/8/22 10:24 AM, Stefan Hajnoczi wrote: > On Tue, Nov 08, 2022 at 09:15:23AM -0700, Jens Axboe wrote: >> On 11/8/22 9:10 AM, Stefan Hajnoczi wrote: >>> On Tue, Nov 08, 2022 at 07:09:30AM -0700, Jens Axboe wrote: >>>> On 11/8/22 7:00 AM, Stefan Hajnoczi wrote: >>>>> On Mon, Nov 07, 2022 at 02:38:52PM -0700, Jens Axboe wrote: >>>>>> On 11/7/22 1:56 PM, Stefan Hajnoczi wrote: >>>>>>> Hi Jens, >>>>>>> NICs and storage controllers have interrupt mitigation/coalescing >>>>>>> mechanisms that are similar. >>>>>> >>>>>> Yep >>>>>> >>>>>>> NVMe has an Aggregation Time (timeout) and an Aggregation Threshold >>>>>>> (counter) value. When a completion occurs, the device waits until the >>>>>>> timeout or until the completion counter value is reached. >>>>>>> >>>>>>> If I've read the code correctly, min_wait is computed at the beginning >>>>>>> of epoll_wait(2). NVMe's Aggregation Time is computed from the first >>>>>>> completion. >>>>>>> >>>>>>> It makes me wonder which approach is more useful for applications. With >>>>>>> the Aggregation Time approach applications can control how much extra >>>>>>> latency is added. What do you think about that approach? >>>>>> >>>>>> We only tested the current approach, which is time noted from entry, not >>>>>> from when the first event arrives. I suspect the nvme approach is better >>>>>> suited to the hw side, the epoll timeout helps ensure that we batch >>>>>> within xx usec rather than xx usec + whatever the delay until the first >>>>>> one arrives. Which is why it's handled that way currently. That gives >>>>>> you a fixed batch latency. >>>>> >>>>> min_wait is fine when the goal is just maximizing throughput without any >>>>> latency targets. >>>> >>>> That's not true at all, I think you're in different time scales than >>>> this would be used for. >>>> >>>>> The min_wait approach makes it hard to set a useful upper bound on >>>>> latency because unlucky requests that complete early experience much >>>>> more latency than requests that complete later. >>>> >>>> As mentioned in the cover letter or the main patch, this is most useful >>>> for the medium load kind of scenarios. For high load, the min_wait time >>>> ends up not mattering because you will hit maxevents first anyway. For >>>> the testing that we did, the target was 2-300 usec, and 200 usec was >>>> used for the actual test. Depending on what the kind of traffic the >>>> server is serving, that's usually not much of a concern. From your >>>> reply, I'm guessing you're thinking of much higher min_wait numbers. I >>>> don't think those would make sense. If your rate of arrival is low >>>> enough that min_wait needs to be high to make a difference, then the >>>> load is low enough anyway that it doesn't matter. Hence I'd argue that >>>> it is indeed NOT hard to set a useful upper bound on latency, because >>>> that is very much what min_wait is. >>>> >>>> I'm happy to argue merits of one approach over another, but keep in mind >>>> that this particular approach was not pulled out of thin air AND it has >>>> actually been tested and verified successfully on a production workload. >>>> This isn't a hypothetical benchmark kind of setup. >>> >>> Fair enough. I just wanted to make sure the syscall interface that gets >>> merged is as useful as possible. >> >> That is indeed the main discussion as far as I'm concerned - syscall, >> ctl, or both? At this point I'm inclined to just push forward with the >> ctl addition. A new syscall can always be added, and if we do, then it'd >> be nice to make one that will work going forward so we don't have to >> keep adding epoll_wait variants... > > epoll_wait3() would be consistent with how maxevents and timeout work. > It does not suffer from extra ctl syscall overhead when applications > need to change min_wait. > > The way the current patches add min_wait into epoll_ctl() seems hacky to > me. struct epoll_event was meant for file descriptor event entries. It > won't necessarily be large enough for future extensions (luckily > min_wait only needs a uint64_t value). It's turning epoll_ctl() into an > ioctl()/setsockopt()-style interface, which is bad for anything that > needs to understand syscalls, like seccomp. A properly typed > epoll_wait3() seems cleaner to me. The ctl method is definitely a bit of an oddball. I've highlighted why I went that way in earlier emails, but in summary: - Makes it easy to adopt, just adding two lines at init time. - Moves detection of availability to init time as well, rather than the fast path. I don't think anyone would want to often change the wait, it's something you'd set at init time. If you often want to change values for some reason, then obviously a syscall parameter would be a lot better. epoll_pwait3() would be vastly different than the other ones, simply because epoll_pwait2() is already using the maximum number of args. We'd need to add an epoll syscall struct at that point, probably with flags telling us if signal_struct or timeout is actually valid. This is not to say I don't think we should add a syscall interface, just some of the arguments pro and con from having actually looked at it. -- Jens Axboe