On Sunday 13 July 2014 at 03:33:57, Edward Shishkin wrote: > On 07/09/2014 02:40 PM, Ivan Shapovalov wrote: > > On Monday 07 July 2014 at 01:47:41, Edward Shishkin wrote: > >> On 06/22/2014 12:48 PM, Ivan Shapovalov wrote: > >> [...] > [...] > >> > >>> + * - if a single extent is smaller than the erase unit, then this particular > >>> + * extent won't be discarded even if it is surrounded by enough free blocks > >>> + * to constitute a whole erase unit; > >> > >> Why not to discard the aligned and padded extent, which coincides > >> with the whole erase unit? > > With a number of whole erase units. > > > >> > >>> + * - we won't be able to merge small adjacent extents forming an extent long > >>> + * enough to be discarded. > >> > >> At this point we have already sorted and merged everything. > >> So may be it makes sense just to check the head and tail of every resulted > >> extent and discard the aligned and padded one? > > "Head and tail" is not sufficient. We may check the whole extent with a single > > bitmap request, but such algorithm will be very inefficient: it will miss many > > possibilities for discarding. > > > > Consider many-block extent, from which one block has been allocated again. > > In this case we miss (all-1) blocks to be discarded (if granularity = 1 block). > > > >> Please, consider such possibility. Iterating over erase units in > >> discard_extent() > >> looks suboptimal. > > Yes, it's costly... but I don't see any other ways to do the task efficiently. > > > How about this function? (the attached patch is against v6-series). > Total number of bitmap checks is in [N+1, 2N], where N is number of > extents in the list. At the same time we don't leave any non-discarded > "garbage"... > > Edward. > P.S. I tested it, but not enough. Hm. I'm probably a dumbass, but... I don't see where the [start; start+len) region is checked for being free. Also, btw, do we need to cut the head (lines 155-163 of the patch) if headp is empty? It seems that it would reduce extent by one whole erase unit without any justification. But I like the idea of gluing padded extents together if possible... Thanks, -- Ivan Shapovalov / intelfx /
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