Lukas Czerner <lczerner@xxxxxxxxxx> writes: > On Mon, 26 Apr 2010, Ric Wheeler wrote: > >> On 04/26/2010 01:46 PM, Lukas Czerner wrote: >> > On Mon, 26 Apr 2010, Jan Kara wrote: >> > >> > > > On Wed, 21 Apr 2010, Greg Freemyer wrote: >> > > > And also, currently I am rewriting the patch do use rbtree instead of >> > > > the >> > > > bitmap, because there were some concerns of memory consumption. It is a >> > > > question whether or not the rbtree will be more memory friendly. >> > > > Generally I think that in most "normal" cases it will, but there are >> > > > some >> > > > extreme scenarios, where the rbtree will be much worse. Any comment on >> > > > this ? >> > > I see two possible improvements here: >> > > a) At a cost of some code complexity, you can bound the worst case by >> > > combining >> > > RB-trees with bitmaps. The basic idea is that when space to TRIM gets too >> > > fragmented (memory to keep to-TRIM blocks in RB-tree for a given group >> > > exceeds >> > > the memory needed to keep it in a bitmap), you convert RB-tree for a >> > > problematic group to a bitmap and attach it to an appropriate RB-node. If >> > > you >> > > track with a bitmap also a number of to-TRIM extents in the bitmap, you >> > > can >> > > also decide whether it's benefitial to switch back to an RB-tree. >> > >> > This sounds like a good idea, but I wonder if it is worth it : >> > 1. The tree will have very short life, because with next ioctl all >> > stored deleted extents will be trimmed and removed from the tree. >> > 2. Also note, that the longer it lives the less fragmented it possibly >> > became. >> > 3. I do not expect, that deleted ranges can be too fragmented, and >> > even if it is, it will be probably merged into one big extent very >> > soon. >> > >> > > >> > > b) Another idea might be: When to-TRIM space is fragmented (again, let's >> > > say >> > > in some block group), there's not much point in sending tiny trim commands >> > > anyway (at least that's what I've understood from this discussion). So you >> > > might as well stop maintaining information which blocks we need to trim >> > > for that group. When the situation gets better, you can always walk block >> > > bitmap and issue trim commands. You might even trigger this rescan from >> > > kernel - if you'd maintain number of free block extents for each block >> > > group >> > > (which is rather easy), you could trigger the bitmap rescan and trim as >> > > soon >> > > as ratio number of free blocks / number of extents gets above a reasonable >> > > threshold. >> > > >> > > Honza >> > > >> > >> > In what I am preparing now, I simple ignore small extents, which would >> > be created by splitting the deleted extent into smaller pieces by chunks >> > of used blocks. This, in my opinion, will prevent the fragmentation, >> > which otherwise may occur in the longer term (between ioctl calls). >> > >> > Thanks for suggestions. >> > -Lukas >> >> I am not convinced that ignoring small extents is a good idea. Remember that >> for SSD's specifically, they remap *everything* internally so our >> "fragmentation" set of small spaces could be useful for them. >> >> That does not mean that we should not try to send larger requests down to the >> target device which is always a good idea I think :-) >> >> ric >> > > That's right, so the other approach would be probably better. Merge > small extents together into one, but there must be some limit, because I > do not want two little extents at the beginning and the end of the group > to force trimming whole group. The whole rbtree thing gets a little > complicated :) This discussion is getting a bit too abstract for me. Show us the code and we can make some progress. =) On the topic of discarding small blocks, I agree with Ric, it should be done. Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
