On Jun 13, 2017, at 12:26 PM, Jens Axboe <axboe@xxxxxxxxx> wrote: > > On 06/13/2017 12:04 PM, Andreas Dilger wrote: >> On Jun 13, 2017, at 11:15 AM, Jens Axboe <axboe@xxxxxxxxx> wrote: >>> >>> A new iteration of this patchset, previously known as write streams. >>> Instead of exposing numeric values for streams, I've previously >>> advocated for just doing a set of hints that makes sense instead. See >>> the coverage from the LSFMM summit this year: >>> >>> https://lwn.net/Articles/717755/ >>> >>> This patchset attempts to do that. We define 4 flags for the pwritev2 >>> system call: >>> >>> RWF_WRITE_LIFE_SHORT Data written with this flag is expected to have >>> a high overwrite rate, or life time. >>> >>> RWF_WRITE_LIFE_MEDIUM Longer life time than SHORT >>> >>> RWF_WRITE_LIFE_LONG Longer life time than MEDIUM >>> >>> RWF_WRITE_LIFE_EXTREME Longer life time than LONG >>> >>> The idea is that these are relative values, so an application can >>> use them as they see fit. The underlying device can then place >>> data appropriately, or be free to ignore the hint. It's just a hint. >>> >>> Comments appreciated. >> >> I thought that one of the major attractions of numeric stream IDs was >> that they had no semantic meanings, just "N is similar to N" and "M is >> similar to M", and it is up to userspace to define what these mean? >> >> That allows userspace to use the IDs for lifetimes (as above), but >> also/instead use them for allocation sizes, different applications, >> different users, etc. > > Right, that is indeed the intent. But we have to attach some naming > to them. Userspace could in theory use these totally randomly, and > things like NVMe would not care. But the semantic meaning of "short" > vs "long" is important on caching infrastructure where you might > want to use the hint for data placement. > > I think the important part here is that no absolute meaning is > attached to them, only relative. In both IOCB_WRITE_LIFE_* and RWF_WRITE_LIFE_* this is consuming 4 bits of space (which is itself fine) for only 4 different stream IDs. Why not just shift a 4-bit arbitrary stream ID to the appropriate offset in those fields, rather than treating them as 4 individual bits and allowing only one of them to be passed down the stack at a time? Cheers, Andreas
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