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[2403:580d:fda1::299]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-82f8e9fbb85sm23252669b3a.22.2026.04.23.02.54.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Apr 2026 02:54:07 -0700 (PDT) Message-ID: <5e19c224-b007-43f9-aa73-3016b5e8ce2c@suse.com> Date: Thu, 23 Apr 2026 19:24:02 +0930 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] btrfs: Limit size of bios submitted from writeback To: Jan Kara , David Sterba Cc: linux-btrfs@vger.kernel.org References: <20260423093052.13219-2-jack@suse.cz> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=wqu@suse.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNGFF1IFdlbnJ1byA8d3F1QHN1c2UuY29tPsLAlAQTAQgAPgIbAwULCQgHAgYVCAkKCwIE FgIDAQIeAQIXgBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXVgBQkQ/lqxAAoJEMI9kfOh Jf6o+jIH/2KhFmyOw4XWAYbnnijuYqb/obGae8HhcJO2KIGcxbsinK+KQFTSZnkFxnbsQ+VY fvtWBHGt8WfHcNmfjdejmy9si2jyy8smQV2jiB60a8iqQXGmsrkuR+AM2V360oEbMF3gVvim 2VSX2IiW9KERuhifjseNV1HLk0SHw5NnXiWh1THTqtvFFY+CwnLN2GqiMaSLF6gATW05/sEd V17MdI1z4+WSk7D57FlLjp50F3ow2WJtXwG8yG8d6S40dytZpH9iFuk12Sbg7lrtQxPPOIEU rpmZLfCNJJoZj603613w/M8EiZw6MohzikTWcFc55RLYJPBWQ+9puZtx1DopW2jOwE0EWdWB rwEIAKpT62HgSzL9zwGe+WIUCMB+nOEjXAfvoUPUwk+YCEDcOdfkkM5FyBoJs8TCEuPXGXBO Cl5P5B8OYYnkHkGWutAVlUTV8KESOIm/KJIA7jJA+Ss9VhMjtePfgWexw+P8itFRSRrrwyUf E+0WcAevblUi45LjWWZgpg3A80tHP0iToOZ5MbdYk7YFBE29cDSleskfV80ZKxFv6koQocq0 vXzTfHvXNDELAuH7Ms/WJcdUzmPyBf3Oq6mKBBH8J6XZc9LjjNZwNbyvsHSrV5bgmu/THX2n g/3be+iqf6OggCiy3I1NSMJ5KtR0q2H2Nx2Vqb1fYPOID8McMV9Ll6rh8S8AEQEAAcLAfAQY AQgAJgIbDBYhBC3fcuWlpVuonapC4cI9kfOhJf6oBQJnEXWBBQkQ/lrSAAoJEMI9kfOhJf6o cakH+QHwDszsoYvmrNq36MFGgvAHRjdlrHRBa4A1V1kzd4kOUokongcrOOgHY9yfglcvZqlJ qfa4l+1oxs1BvCi29psteQTtw+memmcGruKi+YHD7793zNCMtAtYidDmQ2pWaLfqSaryjlzR /3tBWMyvIeWZKURnZbBzWRREB7iWxEbZ014B3gICqZPDRwwitHpH8Om3eZr7ygZck6bBa4MU o1XgbZcspyCGqu1xF/bMAY2iCDcq6ULKQceuKkbeQ8qxvt9hVxJC2W3lHq8dlK1pkHPDg9wO JoAXek8MF37R8gpLoGWl41FIUb3hFiu3zhDDvslYM4BmzI18QgQTQnotJH8= In-Reply-To: <20260423093052.13219-2-jack@suse.cz> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 在 2026/4/23 19:00, Jan Kara 写道: > Currently btrfs_writepages() just accumulates as large bio as possible > (within writeback_control constraints) and then submits it. This can > however lead to significant latency in writeback IO submission (I have > observed tens of miliseconds) because the submitted bio easily has over > hundred of megabytes. Consequently this leads to IO pipeline stalls and > reduced throughput. > > At the same time beyond certain size submitting so large bio provides > diminishing returns because the bio is split by the block layer > immediately anyway. So compute (estimate of) bio size beyond which we > are unlikely to improve performance and just submit the bio for > writeback once we accumulate that much to keep the IO pipeline busy. > This improves writeback throughput for sequential writes by about 15% on > the test machine I was using. > > Signed-off-by: Jan Kara Looks great! Reviewed-by: Qu Wenruo Just one minor question inlined below. [...] > +int btrfs_init_writeback_bio_size(struct btrfs_fs_info *fs_info) > +{ > + struct btrfs_fs_devices *fs_devices = fs_info->fs_devices; > + struct btrfs_device *device; > + u32 writeback_bio_size = fs_info->sectorsize; > + > + mutex_lock(&fs_devices->device_list_mutex); > + /* > + * Let's take maximum over optimal request sizes for all devices. For > + * RAID profiles writeback will submit stripe (64k) sized bios anyway > + * so our value doesn't matter and for simple profiles this is a good > + * approximation of sensible IO chunking. > + */ > + list_for_each_entry(device, &fs_devices->devices, dev_list) { > + struct request_queue *queue; > + unsigned int io_opt; > + > + queue = bdev_get_queue(device->bdev); > + io_opt = queue_io_opt(queue) ? : > + queue_max_sectors(queue) << SECTOR_SHIFT; > + writeback_bio_size = max(writeback_bio_size, io_opt); > + } > + mutex_unlock(&fs_devices->device_list_mutex); > + > + fs_info->writeback_bio_size = writeback_bio_size; With this simplified version of optimal io size detection, do we want to hook dev add/removal/replace to update the calculation? I guess in the real world, the added/removed/replaced disks should have all the same performance parameter for server usages, so no difference there. And for personal/pro users, I doubt if the original performance problem is even noticeable for most end users. So overall I'm fine either way. Thanks, Qu