Messages in this thread | | | Date | Sat, 18 Apr 2015 13:53:14 -0600 | From | Jens Axboe <> | Subject | Re: [PATCH 1/7] block: add support for carrying a stream ID in a bio |
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On 04/09/2015 04:46 PM, Andreas Dilger wrote: > On Mar 25, 2015, at 9:07 AM, Jens Axboe <axboe@fb.com> wrote: >> >> The top bits of bio->bi_flags are reserved for keeping the >> allocation pool, set aside the next eight bits for carrying >> a stream ID. That leaves us with support for 255 streams, >> 0 is reserved as a "stream not set" value. > > I understand that the stream ID is not related to specific priority > of an IO request. However, I'm wondering how this patch series > interacts with some of the other patch series that add cache priority > hints? Is there a danger of running out of space in the IO pipeline > for the additional cache hints if this is using 8 bits?
That's always a risk, of course, but that goes for most features that need to carry more data in struct bio (or elsewhere). Otherwise we'll have to bite the bullet and add a new field.
>> +/* >> + * after the pool bits, next 8 bits are for the stream id >> + */ >> +#define BIO_STREAM_BITS (8) >> +#define BIO_STREAM_OFFSET (BITS_PER_LONG - 12) > > Should this really be: > > #define BIO_STREAM_OFFSET (BIO_POOL_OFFSET - BIO_STREAM_BITS) > > Otherwise there is a risk of conflict if someone changes BIO_POOL_BITS.
Good point, that would be cleaner. I'll make that change.
-- Jens Axboe
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