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Date:   Fri, 08 Nov 2019 18:21:56 +0300
From:   Dmitry Monakhov <dmonakhov@...il.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: fix extent_status fragmentation for plain files


"Theodore Y. Ts'o" <tytso@....edu> writes:

> On Wed, Nov 06, 2019 at 12:25:02PM +0000, Dmitry Monakhov wrote:
>> It is appeared that extent are not cached for inodes with depth == 0
>> which result in suboptimal extent status populating inside ext4_map_blocks()
>> by map's result where size requested is usually smaller than extent size so
>> cache becomes fragmented
>> 
>> # Example: I have plain file:
>> File size of /mnt/test is 33554432 (8192 blocks of 4096 bytes)
>>  ext:     logical_offset:        physical_offset: length:   expected: flags:
>>    0:        0..    8191:      40960..     49151:   8192:             last,eof
>> 
>> $ perf record -e 'ext4:ext4_es_*' /root/bin/fio --name=t --direct=0 --rw=randread --bs=4k --filesize=32M --size=32M --filename=/mnt/test
>> $ perf script | grep ext4_es_insert_extent | head -n 10
>>              fio   131 [000]    13.975421:           ext4:ext4_es_insert_extent: dev 253,0 ino 12 es [494/1) mapped 41454 status W
>>              fio   131 [000]    13.976467:           ext4:ext4_es_insert_extent: dev 253,0 ino 12 es [6907/1) mapped 47867 status W
>
> So this is certainly bad behavior, but the original intent was to not
> cached extents that were in the inode's i_blocks[] array because the
> information was already in the inode cache, and so we could save
> memory but just pulling the information out of the i_blocks away and
> there was no need to cache the extent in the es cache.
>
> There are cases where we do need to track the extent in the es cache
> --- for example, if we are writing the file and we need to track its
> delayed allocation status.
>
> So I wonder if we might be better off defining a new flag
> EXT4_MAP_INROOT, which gets set by ext4_ext_map_blocks() and
> ext4_ind_map_blocks() if the mapping is exclusively found in the
> i_blocks array, and if EXT4_MAP_INROOT is set, and we don't need to
> set EXTENT_STATUS_DELAYED, we skip the call to
> ext4_es_insert_extent().
>
> What do you think?  This should significantly reduce the memory
> utilization of the es_cache, which would be good for low-memory
> workloads, and those where there are a large number of inodes that fit
> in the es_cache, which is probably true for most desktops, especially
> those belonging kernel developers.  :-)

Sound reasonable, same happens in ext4_ext_precache()
See my patch below.

But this also means that on each ext4_map_blocks()
will fallback to regular block lookup:

down_read(&EXT4_I(inode)->i_data_sem)
ext4_ext_map_blocks (
 ->ext4_find_extent
   path = kcalloc(depth + 2, sizeof(struct ext4_ext_path), GFP_NOFS)
   kfree(path)
up_read(&EXT4_I(inode)->i_data_sem)
I thought that we can neglect this, but curiosity is a good thing
That it why I've tested nocache(see patch below) vs cache(first path)  approach

## Production server ##
# CPU: Intel-Gold-6230
# DEV: /dev/ram0 
# TEST: fio --direct=1 --rw=randread --bs=1k --filesize=64M
IOPS(nocache/cache): 729k vs 764k  => +5%
# DEV: /dev/nvme0n1 (Samsung DC grade)
# TEST: fio --direct=1 --rw=randread --bs=4k --filesize=64M --ioengine=libaio --iodepth=128
IOPS(nocache/cache): 366k vs 378k => +3%

## My notebook Carbon/X1 ##
# CPU: i7-7600U
# DEV: /dev/nvme0n1 (Samsung MZVLB512HAJQ-000L7)
# TEST: fio --direct=1 --rw=randread --bs=4k --filesize=64M --ioengine=libaio --iodepth=128
IOPS(nocache/cache): 270k vs 270k => No difference

Difference is invisiable for my laptop, but visiable on fast NVME dev.
>From other point of view unconditional caching result in memory
overhead  sizeof(struct extent_status)/sizeof(ext4_inode_info) => 3.5% for everybody.
>
> 						- Ted

View attachment "0001-ext4-Don-t-cache-clean-inline-extents.patch" of type "text/x-diff" (1293 bytes)

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