[RFC PATCH 0/1] erofs: change to use asynchronous io for fscache readahead

[Xin Yin]

Hi Jeffle & Xiang

I have tested your fscache,erofs: fscache-based on-demand read semantics 
v9 patches sets https://www.spinics.net/lists/linux-fsdevel/msg216178.html.
For now , it works fine with the nydus image-service. After the image data 
is fully loaded to local storage, it does have great IO performance gain 
compared with nydus V5 which is based on fuse.

For 4K random read , fscache-based erofs can get the same performance with 
the original local filesystem. But I still saw a performance drop in the 4K 
sequential read case. And I found the root cause is in erofs_fscache_readahead() 
we use synchronous IO , which may stall the readahead pipelining.

I have tried to change to use asynchronous io during erofs fscache readahead 
procedure, as what netfs did. Then I saw a great performance gain.

Here are my test steps and results:
- generate nydus v6 format image , in which stored a large file for IO test.
- launch nydus image-service , and  make image data fully loaded to local storage (ext4).
- run fio with below cmd.
fio -ioengine=psync -bs=4k -size=5G -direct=0 -thread -rw=read -filename=./test_image  -name="test" -numjobs=1 -iodepth=16 -runtime=60

v9 patches: 202654 KB/s
v9 patches + async readahead patch: 407213 KB/s
ext4: 439912 KB/s


Xin Yin (1):
  erofs: change to use asynchronous io for fscache readahead

 fs/erofs/fscache.c | 256 ++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 245 insertions(+), 11 deletions(-)

-- 
2.11.0