On 1/16/23 3:52 AM, Breno Leitao wrote: > On Thu, Jan 12, 2023 at 05:35:22PM -0300, Gabriel Krisman Bertazi wrote: >> Breno Leitao <leitao@xxxxxxxxxx> writes: >> >>> This patch removes some "cold" fields from `struct io_issue_def`. >>> >>> The plan is to keep only highly used fields into `struct io_issue_def`, so, >>> it may be hot in the cache. The hot fields are basically all the bitfields >>> and the callback functions for .issue and .prep. >>> >>> The other less frequently used fields are now located in a secondary and >>> cold struct, called `io_cold_def`. >>> >>> This is the size for the structs: >>> >>> Before: io_issue_def = 56 bytes >>> After: io_issue_def = 24 bytes; io_cold_def = 40 bytes >> >> Does this change have an observable impact in run time? Did it show >> a significant decrease of dcache misses? > > I haven't tested it. I expect it might be hard to came up with such test. > > A possible test might be running io_uring heavy tests, while adding > enough memory pressure. Doing this in two different instant (A/B test), > might be a unpredicable and the error deviation might hide the benefit. I think what you'd want is two (or more) io_uring ops being really busy and measuring dcache pressure while running that test. I don't think this is very feasible to accurately measure, and I also don't think that is an issue. The split into hot/cold parts of the op definitions is obviously a good idea. For ideal setups, we'll never be using the cold part at all, and having a smaller op definition for the fast path is always going to be helpful. -- Jens Axboe
