On Thu, Sep 09, 2021 at 09:05:13PM -0600, Jens Axboe wrote: > On 9/9/21 8:57 PM, Al Viro wrote: > > On Thu, Sep 09, 2021 at 03:19:56PM -0600, Jens Axboe wrote: > > > >> Not sure how we'd do that, outside of stupid tricks like copy the > >> iov_iter before we pass it down. But that's obviously not going to be > >> very efficient. Hence we're left with having some way to reset/reexpand, > >> even in the presence of someone having done truncate on it. > > > > "Obviously" why, exactly? It's not that large a structure; it's not > > the optimal variant, but I'd like to see profiling data before assuming > > that it'll cause noticable slowdowns. > > It's 48 bytes, and we have to do it upfront. That means we'd be doing it > for _all_ requests, not just when we need to retry. As an example, current > benchmarks are at ~4M read requests per core. That'd add ~200MB/sec of > memory traffic just doing this copy. Umm... How much of that will be handled by cache? > Besides, I think that's moot as there's a better way. I hope so, but I'm afraid that "let's reload from userland on e.g. short reads" is not better - there's a plenty of interesting corner cases you need to handle with that.
