Hi, Matthew
On 2023/10/31 上午12:47, Matthew Wilcox wrote:
On Mon, Oct 30, 2023 at 03:41:30PM +0800, Youling Tang wrote:
From: Youling Tang <tangyouling@xxxxxxxxxx>
Changing the read_ahead_kb value midway through a sequential read of a
large file found that the ra->ra_pages value remained unchanged (new
ra_pages can only be detected the next time the file is opened). Because
file_ra_state_init() is only called once in do_dentry_open() in most
cases.
In ondemand_readahead(), update bdi->ra_pages to ra->ra_pages to ensure
that the maximum pages that can be allocated by the readahead algorithm
are the same as (read_ahead_kb * 1024) / PAGE_SIZE after read_ahead_kb
is modified.
Explain to me why this is the correct behaviour.
Because I initially expected to immediately improve the current read
performance
by modifying read_ahead_kb when reading large files sequentially.
Many things are only initialised at open() time and are not updated until
the next open(). This is longstanding behaviour that some apps expect.
Thanks for your explanation. I will discard this change if the next open
update is in line with the
apps expectation.
Thanks,
Youling.
