Re: Expense of read_iter

[Mikulas Patocka]

On Wed, 20 Jan 2021, Jan Kara wrote:

> Yeah, I agree. I'm against ext4 private solution for this read problem. And
> I'm also against duplicating ->read_iter functionatily in ->read handler.
> The maintenance burden of this code duplication is IMHO just too big. We
> rather need to improve the generic code so that the fast path is faster.
> And every filesystem will benefit because this is not ext4 specific
> problem.
> 
> 								Honza

Do you have some idea how to optimize the generic code that calls 
->read_iter?

vfs_read calls ->read if it is present. If not, it calls new_sync_read. 
new_sync_read's frame size is 128 bytes - it holds the structures iovec, 
kiocb and iov_iter. new_sync_read calls ->read_iter.

I have found out that the cost of calling new_sync_read is 3.3%, Zhongwei 
found out 3.9%. (the benchmark repeatedy reads the same 4k page)

I don't see any way how to optimize new_sync_read or how to reduce its 
frame size. Do you?

Mikulas