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Date:   Tue, 12 Jan 2021 16:45:29 +0100
From:   David Sterba <dsterba@...e.cz>
To:     kernel test robot <oliver.sang@...el.com>
Cc:     Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        René Rebe <rene@...ctcode.de>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...el.com, linux-btrfs@...r.kernel.org
Subject: Re: [btrfs]  e076ab2a2c:  fio.write_iops -18.3% regression

On Tue, Jan 12, 2021 at 11:36:14PM +0800, kernel test robot wrote:
> 
> Greeting,
> 
> FYI, we noticed a -18.3% regression of fio.write_iops due to commit:
> 
> 
> commit: e076ab2a2ca70a0270232067cd49f76cd92efe64 ("btrfs: shrink delalloc pages instead of full inodes")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> 
> in testcase: fio-basic
> on test machine: 192 threads Intel(R) Xeon(R) CPU @ 2.20GHz with 192G memory
> with following parameters:
> 
> 	disk: 1SSD
> 	fs: btrfs
> 	runtime: 300s
> 	nr_task: 8
> 	rw: randwrite
> 	bs: 4k
> 	ioengine: sync
> 	test_size: 256g

Though I do a similar test (emulating bit torrent workload), it's a bit
extreme as it's 4k synchronous on a huge file. It always takes a lot of
time but could point out some concurrency issues namely on faster
devices. There are 8 threads possibly competing for the same inode lock
or other locks related to it.

The mentioned commit fixed another perf regression on a much more common
workload (untgrring files), so at this point drop in this fio workload
is inevitable.

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