On 8/6/21 2:48 PM, Al Viro wrote: > On Fri, Aug 06, 2021 at 12:42:43PM +0100, Pavel Begunkov wrote: >> 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. > >> 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
