linux-ext4 - Re: mmc filesystem performance decreased on the first write after filesystem creation

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Date:   Wed, 30 May 2018 11:44:04 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Christoph Hellwig <hch@....de>, Faiz Abbas <faiz_abbas@...com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Jens Axboe <axboe@...nel.dk>, linux-ext4@...r.kernel.org,
        tytso@....edu
Subject: Re: mmc filesystem performance decreased on the first write after
 filesystem creation

On 28/05/18 09:26, Christoph Hellwig wrote:
> Summary: mke2s uses the BLKDISCARD ioctl to wipe the device,
> and then uses BLKDISCARDZEROES to check if that zeroed the data.
> 
> A while ago I made BLKDISCARDZEROES always return 0 because it is
> basically impossible to have reliably zeroing using discard as the
> standards leave the devices way to many options to not actually
> zero data at their own choice when using the discard commands.

Older eMMC do not have a "discard" option and use "erase" instead.  "Erase"
has similar benefits to "discard" but the eMMC is required to make the
erased blocks read as either all 0's or all 1's.

> 
> So IFF mke2fs want to actually free space and zero it it needs
> to use fallocate to punch a hole, and mmc needs to implement
> REQ_OP_WRITE_ZEROS IFF it actually has a reliable way to zero
> blocks.
> 
> 
> On Tue, May 22, 2018 at 08:48:31PM +0530, Faiz Abbas wrote:
>> Hi,
>>
>> I am debugging a performance reduction in ext2 filesystems on an mmc
>> device in TI's am335x evm board.
>>
>> I see that the performance is reduced on the first write after making a
>> new filesystem using mkfs.ext2 on one of the mmc partitions. The
>> performance comes back to normal after the first write.
>>
>> commands used:
>>
>> => umount /dev/mmcblk1p2
>>
>> => mkfs.ext2 -F  /dev/mmcblk1p2
>>
>> => mount -t ext2 -o async /dev/mmcblk1p2 /mnt/partition_mmc
>>
>> => dd if=/dev/urandom of=/dev/shm/srctest_file_mmc_1184 bs=1M count=10
>>
>> => ./filesystem_tests -write -src_file /dev/shm/srctest_file_mmc_1184
>> -srcfile_size 10 -file /mnt/partition_mmc/test_file_1184 -buffer_size
>> 102400 -file_size 100 -performance
>>
>> The filesystem_tests write utility reads from the file generated at
>> /dev/shm/srctest_file_mmc_1184, memory maps the file to a buffer, and
>> then writes it into the newly created /mnt/partition_mmc in multiples of
>> buffer_size while measuring write performance.
>>
>> See here for the implementation of filesystem_tests write utility:
>> http://arago-project.org/git/projects/?p=test-automation/ltp-ddt.git;a=blob;f=testcases/ddt/filesystem_test_suite/src/testcases/st_filesystem_write_to_file.c;h=80e8e244d7eaa9f0dbd9b21ea705445156c36bef;hb=f7fc06c290333ce08a7d4fba104eee0f0f1d942b
>>
>> Complete log with multiple calls to filesystem_tests:
>> https://pastebin.ubuntu.com/p/BckmTJpqPv/
>>
>> Notice that the first run of filesystem_tests has a lower throughput
>> reported.
>>
>> I was able to bisect the issue to this commit:
>> 5d1429fead5b (mmc: remove the discard_zeroes_data flag)
>>
>> I would assume that after this flag is removed, the filesystem creation
>> command would explicitly write zeroes to the device which might explain
>> the performance fall. However, then the mkfs.ext2 command itself should
>> take more time rather than the first file write after that.

You might want to check the lazy initialization options.  I always use
"-Elazy_itable_init=0,lazy_journal_init=0" with ext4 to prevent it messing
up performance tests.

>>
>> It would be nice if someone could help me understand why this is happening.
>>
>> Thanks for your help.
>>
>> Regards,
>> Faiz
> ---end quoted text---
>