[LU-12043] improve Lustre single thread read performances

Duplicate — Sat, 29 Jan 2022 10:04:13 +0000

There are several efforts here and there which try to improve the performances of single
thread read performances.

this ticket is opened to track a simple enough patch to improve the performances as much
as possible.

Here is whole history:

Currently, for sequential read IO, We grow up window
size very quickly, and once we cached @max_readahead_per_file
pages. For following command:

dd if=/mnt/lustre/file of=/dev/null bs=1M

We will do something like following:
...
64M bytes cached.
fast io for 16M bytes
readahead extra 16M to fill up window.
fast io for 16M bytes
readahead extra 16M to fill up window.
....

In this way, we could only use fast IO for 16M bytes and
then fall through non-fast IO mode. this is also reason
that why increasing @max_readahead_per_file don't give us
performances up, since this value only changes how much
memory we cached in memory, during my testing whatever
I changed the value, i could only get 2GB/s for single thread
read.

Actually, we could do this better, if we have used
more than 16M bytes readahead pages, submit another readahead
requests in the background. and ideally, we could always
use fast IO..I did a quick test with fake-IO on my limited
PC server:

Without patch VS Patched:
~2.0GB/S vs ~3.0 GB/s

So we could gain at least 50% performance up, i supposed
We could get more maybe more than 4GB/S with patch.

Whamcloud Community JIRA

[LU-12043] improve Lustre single thread read performances