Re: [RFC] mm: optimise generic_file_read_iter

On 8/6/21 2:48 PM, Al Viro wrote:

On Fri, Aug 06, 2021 at 12:42:43PM +0100, Pavel Begunkov wrote:

quoted

Unless direct I/O path of generic_file_read_iter() ended up with an
error or a short read, it doesn't use inode. So, load inode and size
later, only when they're needed. This cuts two memory reads and also
imrpoves code generation, e.g. loads from stack.

... and the same question here.

quoted

NOTE: as a side effect, it reads inode->i_size after ->direct_IO(), and
I'm not sure whether that's valid, so would be great to get feedback
from someone who knows better.

Ought to be safe, I think, but again, how much effect have you observed
from the patch?

Answering for both patches -- I haven't benchmarked it and don't expect
to find anything just from this one, considering variance between runs.
I took a loot at the assembly (gcc 11.1), it removes 2 reads to get
i_size, write+read that i_size from stack, because it stashed it on
the stack.

For example, we've squeezed several percents of throughput before on
the io_uring side just by cutting sheer number of not too expensive
individually instructions. IMHO, it's easier to do when you spotted
something by the way, than rediscovering the same during a performance
safari.

-- 
Pavel Begunkov