On Mon, Nov 24 2008, James Bottomley wrote: > On Mon, 2008-11-24 at 19:57 +0100, Jens Axboe wrote: > > On Mon, Nov 24 2008, David Woodhouse wrote: > > > On Mon, 2008-11-24 at 13:42 -0500, James Bottomley wrote: > > > > On Mon, 2008-11-24 at 09:03 +0000, David Woodhouse wrote: > > > > > On Mon, 2008-11-24 at 07:52 +0900, James Bottomley wrote: > > > > > > On Sun, 2008-11-23 at 13:39 +0000, David Woodhouse wrote: > > > > > > > > We don't attempt to put non-contiguous ranges into a single TRIM yet. > > > > > > > > > > > > > > We don't even merge contiguous ranges -- I still need to fix the > > > > > > > elevators to stop writes crossing writes, > > > > > > > > > > > > I don't think we want to do that ... it's legal if the write isn't a > > > > > > barrier and it will inhibit merging. That may be just fine for a SSD, > > > > > > but it's not for spinning media since they get better performance out of > > > > > > merged writes. > > > > > > > > > > No, I just mean writes _to the same sector_. At the moment, we happily > > > > > let those cross each other in the queue. > > > ... > > > > It's not a bug ... but changing it might be feasible ... as long as it > > > > doesn't affect write performance too much (which I don't think it will), > > > > since it is in the critical path. > > > > > > We could argue about how much sense it makes to let two writes to the > > > same sector actually happen in reverse order. > > > > > > Especially given the fact that we actually _do_ preserve ordering in > > > some cases; just not in others. (We preserve ordering only if the start > > > and end of the duplicate writes are _precisely_ matching; if it's just > > > overlapping (which may well happen in the presence of merges), then this > > > check doesn't trigger. > > > > > > But that's just semantics. Yes, changing it should be feasible. I talked > > > to Jens about that at the kernel summit, and we agreed that it should > > > probably be done. > > > > The way this currently works is that we sort based on the first sector > > in the request. So if you have have an overlap condition between rq1 and > > rq2 and then a write gets merged into rq1, then you may have passing > > writes. Linux has never guarenteed any write ordering for non-barriers, > > so we've never attempted to handle it. Direct aliases (matching first > > sectors) are handled as you mention, but that's more of an algorithmic > > thing than by design. > > > > My main worry is that this will add considerable overhead to request > > sorting. For the rbtree based sorting, we'd have to do a rb_next/rb_prev > > to look at adjacent requests. For CFQ it's even worse, since there's no > > per-queue big rbtree for sorting. > > Which is why I suggest special casing: Only invoke the expensive > overlap checking if one of the requests is a discard. Otherwise use the > standard path for writes. Good point, we can easily track if we have discard requests pending. That doesn't really make it a lot better for CFQ though, currently it'll still have to dump all queues if a discard comes in. -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
