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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CE06CC433F5 for ; Sun, 26 Sep 2021 10:00:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B7BBE6103B for ; Sun, 26 Sep 2021 10:00:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230514AbhIZKBv (ORCPT ); Sun, 26 Sep 2021 06:01:51 -0400 Received: from out30-57.freemail.mail.aliyun.com ([115.124.30.57]:54086 "EHLO out30-57.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231336AbhIZKBm (ORCPT ); Sun, 26 Sep 2021 06:01:42 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R831e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04395;MF=haoxu@linux.alibaba.com;NM=1;PH=DS;RN=4;SR=0;TI=SMTPD_---0UpbrNVT_1632650404; Received: from B-25KNML85-0107.local(mailfrom:haoxu@linux.alibaba.com fp:SMTPD_---0UpbrNVT_1632650404) by smtp.aliyun-inc.com(127.0.0.1); Sun, 26 Sep 2021 18:00:05 +0800 Subject: Re: [PATCH RFC 5.13 1/2] io_uring: add support for ns granularity of io_sq_thread_idle To: Pavel Begunkov , Jens Axboe Cc: io-uring@vger.kernel.org, Joseph Qi References: <1619616748-17149-1-git-send-email-haoxu@linux.alibaba.com> <1619616748-17149-2-git-send-email-haoxu@linux.alibaba.com> <7136bf4f-089f-25d5-eaf8-1f55b946c005@gmail.com> <51308ac4-03b7-0f66-7f26-8678807195ca@linux.alibaba.com> <96ef70e8-7abf-d820-3cca-0f8aedc969d8@gmail.com> From: Hao Xu Message-ID: <0d781b5f-3d2d-5ad4-9ad3-8fabc994313a@linux.alibaba.com> Date: Sun, 26 Sep 2021 18:00:04 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 MIME-Version: 1.0 In-Reply-To: <96ef70e8-7abf-d820-3cca-0f8aedc969d8@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 在 2021/4/30 上午6:15, Pavel Begunkov 写道: > On 4/29/21 4:28 AM, Hao Xu wrote: >> 在 2021/4/28 下午10:07, Pavel Begunkov 写道: >>> On 4/28/21 2:32 PM, Hao Xu wrote: >>>> currently unit of io_sq_thread_idle is millisecond, the smallest value >>>> is 1ms, which means for IOPS > 1000, sqthread will very likely  take >>>> 100% cpu usage. This is not necessary in some cases, like users may >>>> don't care about latency much in low IO pressure >>>> (like 1000 < IOPS < 20000), but cpu resource does matter. So we offer >>>> an option of nanosecond granularity of io_sq_thread_idle. Some test >>>> results by fio below: >>> >>> If numbers justify it, I don't see why not do it in ns, but I'd suggest >>> to get rid of all the mess and simply convert to jiffies during ring >>> creation (i.e. nsecs_to_jiffies64()), and leave io_sq_thread() unchanged. >> 1) here I keep millisecond mode for compatibility >> 2) I saw jiffies is calculated by HZ, and HZ could be large enough >> (like HZ = 1000) to make nsecs_to_jiffies64() = 0: >> >>  u64 nsecs_to_jiffies64(u64 n) >>  { >>  #if (NSEC_PER_SEC % HZ) == 0 >>          /* Common case, HZ = 100, 128, 200, 250, 256, 500, 512, 1000 etc. */ >>          return div_u64(n, NSEC_PER_SEC / HZ); >>  #elif (HZ % 512) == 0 >>          /* overflow after 292 years if HZ = 1024 */ >>          return div_u64(n * HZ / 512, NSEC_PER_SEC / 512); >>  #else >>          /* >>          ¦* Generic case - optimized for cases where HZ is a multiple of 3. >>          ¦* overflow after 64.99 years, exact for HZ = 60, 72, 90, 120 etc. >>          ¦*/ >>          return div_u64(n * 9, (9ull * NSEC_PER_SEC + HZ / 2) / HZ); >>  #endif >>  } >> >> say HZ = 1000, then nsec_to_jiffies64(1us) = 1e3 / (1e9 / 1e3) = 0 >> iow, nsec_to_jiffies64() doesn't work for n < (1e9 / HZ). > > Agree, apparently jiffies precision fractions of a second, e.g. 0.001s > But I'd much prefer to not duplicate all that. So, jiffies won't do, > ktime() may be ok but a bit heavier that we'd like it to be... > > Jens, any chance you remember something in the middle? Like same source > as ktime() but without the heavy correction it does. I'm gonna pick this one up again, currently this patch with ktime_get_ns() works good on our productions. This patch makes the latency a bit higher than before, but still lower than aio. I haven't gotten a faster alternate for ktime_get_ns(), any hints? Regards, Hao >