On 5/21/18 8:31 AM, Mike Snitzer wrote: > On Mon, May 21 2018 at 10:19am -0400, > Jens Axboe <axboe@xxxxxxxxx> wrote: > >> On 5/21/18 8:03 AM, Mike Snitzer wrote: >>> On Sun, May 20 2018 at 6:25pm -0400, >>> Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: >>> >>>> Jens - this series does the rest of the conversions that Christoph wanted, and >>>> drops bioset_create(). >>>> >>>> Only lightly tested, but the changes are pretty mechanical. Based on your >>>> for-next tree. >>> >>> By switching 'mempool_t *' to 'mempool_t' and 'bio_set *' to 'bio_set' >>> you've altered the alignment of members in data structures. So I'll >>> need to audit all the data structures you've modified in DM. >>> >>> Could we get the backstory on _why_ you're making this change? >>> Would go a long way to helping me appreciate why this is a good use of >>> anyone's time. >> >> Yeah, it's in the first series, it gets rid of a pointer indirection. > > "Allows mempools to be embedded in other structs, getting rid of a > pointer indirection from allocation fastpaths." > > So this is about using contiguous memory or avoiding partial allocation > failure? Or both? > > Or more to it? Just trying to fully appreciate the theory behind the > perceived associated benefit. It's about avoiding a pointer indirection. Instead of having to follow a pointer to get to that struct, it's simple offset math off your main structure. > I do think the increased risk of these embedded bio_set and mempool_t > themselves crossing cachelines, or struct members that follow them doing > so, really detracts from these types of changes. Definitely something to look out for, though most of them should be per-dev structures and not in-flight structures. That makes it a bit less sensitive. But can't hurt to audit the layouts and adjust if necessary. This is why it's posted for review :-) -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
