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. :-) - Ted
