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[109.152.100.164]) by smtp.gmail.com with ESMTPSA id 15sm3176236wmg.1.2020.11.04.10.09.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 04 Nov 2020 10:09:46 -0800 (PST) Subject: Re: [PATCH 1/4] btrfs: discard: speed up discard up to iops_limit To: Amy Parker Cc: Chris Mason , Josef Bacik , David Sterba , Btrfs BTRFS References: <2a3d84dfc9384eed8659963d1dafedabb3f17c75.1604444952.git.asml.silence@gmail.com> <2be31971-da4f-1a39-cb01-0c13b35cf2aa@gmail.com> <4484059b-1e9a-995c-1632-b0ee81eaf605@gmail.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Message-ID: <36897f55-26cf-4814-8549-9392a6e9e4b1@gmail.com> Date: Wed, 4 Nov 2020 18:06:46 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 04/11/2020 17:55, Amy Parker wrote: > On Wed, Nov 4, 2020 at 9:50 AM Pavel Begunkov wrote: >> >> On 04/11/2020 17:33, Amy Parker wrote: >>> On Wed, Nov 4, 2020 at 9:22 AM Pavel Begunkov wrote: >>>> >>>> On 04/11/2020 15:29, Amy Parker wrote: >>>>> On Wed, Nov 4, 2020 at 1:50 AM Pavel Begunkov wrote: >>>>>> >>>>>> Instead of using iops_limit only for cutting off extremes, calculate the >>>>>> discard delay directly from it, so it closely follows iops_limit and >>>>>> doesn't under-discarding even though quotas are not saturated. >>>>> >>>>> This sounds like it potentially be a great performance boost, do you >>>>> have any performance metrics regarding this patch? >>>> >>>> Boosting the discard rate and so reaping stalling blocks may be nice, but >>>> unless it holds too much memory creating lack of space it shouldn't affect >>>> throughput. Though, it's better to ask people with deeper understanding >>>> of the fs. >>> >>> Alright, thanks for the clarification. >>> >>>> What I've seen is that in some cases there are extents staying queued for >>>> discarding for _too_ long. E.g. reaping a small number of very fat extents >>>> keeps delay at max and doesn't allow to reap them effectively. That could >>>> be a problem with fast drives. >>> >>> Ah, yep. Seen this personally to a smaller extent. >>> >>>> >>>>> >>>>>> >>>>>> Signed-off-by: Pavel Begunkov >>>>>> --- >>>>>> fs/btrfs/discard.c | 10 +++++----- >>>>>> 1 file changed, 5 insertions(+), 5 deletions(-) >>>>>> >>>>>> diff --git a/fs/btrfs/discard.c b/fs/btrfs/discard.c >>>>>> index 741c7e19c32f..76796a90e88d 100644 >>>>>> --- a/fs/btrfs/discard.c >>>>>> +++ b/fs/btrfs/discard.c >>>>>> @@ -519,7 +519,6 @@ void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl) >>>>>> s64 discardable_bytes; >>>>>> u32 iops_limit; >>>>>> unsigned long delay; >>>>>> - unsigned long lower_limit = BTRFS_DISCARD_MIN_DELAY_MSEC; >>>>>> >>>>>> discardable_extents = atomic_read(&discard_ctl->discardable_extents); >>>>>> if (!discardable_extents) >>>>>> @@ -550,11 +549,12 @@ void btrfs_discard_calc_delay(struct btrfs_discard_ctl *discard_ctl) >>>>>> >>>>>> iops_limit = READ_ONCE(discard_ctl->iops_limit); >>>>>> if (iops_limit) >>>>>> - lower_limit = max_t(unsigned long, lower_limit, >>>>>> - MSEC_PER_SEC / iops_limit); >>>>>> + delay = MSEC_PER_SEC / iops_limit; >>>>>> + else >>>>>> + delay = BTRFS_DISCARD_TARGET_MSEC / discardable_extents; >>>>> >>>>> Looks good to me. I wonder why there wasn't handling of if iops_limit >>>>> was unfindable >>>>> before? >>>> >>>> Not sure what you mean by unfindable, but async discard is relatively new, >>>> might be that everyone just have their hands full. >>> >>> By unfindable I mean if iops_limit turned up as null when reading it >>> from discard_ctl. >> >> Ahh, ok. It's handled and I left it as it was, that BTW is still a problem. > > How often is iops_limit unfindable? I don't know, but the default is 10, so shouldn't be too ubiquitous. Maybe someone here knows statistics. > >> >> First it calculates a delay based on number of queued extents and than clamps >> it to (BTRFS_DISCARD_MIN_DELAY_MSEC, BTRFS_DISCARD_MAX_DELAY_MSEC). Without >> this patch it did the same but the lower bound was calculated from iops_limit. > > Thanks for clarifying. > >> >>> Async discard was added in 5.6, correct? So yeah, makes sense then that people >>> just had their hands full. Thanks for adding it. >> >> b0643e59cfa609c4b5f ("btrfs: add the beginning of async discard, discard >> workqueue"). Dec 2019, so less than a year > > Thanks for finding the commit. > >> >>> >>>> >>>>> >>>>>> >>>>>> - delay = BTRFS_DISCARD_TARGET_MSEC / discardable_extents; >>>>>> - delay = clamp(delay, lower_limit, BTRFS_DISCARD_MAX_DELAY_MSEC); >>>>>> + delay = clamp(delay, BTRFS_DISCARD_MIN_DELAY_MSEC, >>>>>> + BTRFS_DISCARD_MAX_DELAY_MSEC); >>>>>> discard_ctl->delay = msecs_to_jiffies(delay); >>>>>> >>>>>> spin_unlock(&discard_ctl->lock); >>>>>> -- >>>>>> 2.24.0 >>>>>> >>>>> >>>>> This patch looks all great to me. >> >> -- >> Pavel Begunkov > > Best regards, > Amy Parker > (they/them) > -- Pavel Begunkov