Re: [LKP] Re: [btrfs] e076ab2a2c: fio.write_iops -18.3% regression

[Xing Zhengjun]

On 1/12/2021 11:45 PM, David Sterba wrote:

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.

Do you have a plan to fix it? Thanks.

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--
Zhengjun Xing