(previous incomplete message sent accidentally) On Wednesday 08 August 2007 02:54, Evgeniy Polyakov wrote: > On Tue, Aug 07, 2007 at 10:55:38PM +0200, Jens Axboe wrote: > > So, what did we decide? To bloat bio a bit (add a queue pointer) or > to use physical device limits? The latter requires to replace all > occurence of bio->bi_bdev = something_new with blk_set_bdev(bio, > somthing_new), where queue limits will be appropriately charged. So > far I'm testing second case, but I only changed DST for testing, can > change all other users if needed though. Adding a queue pointer to struct bio and using physical device limits as in your posted patch both suffer from the same problem: you release the throttling on the previous queue when the bio moves to a new one, which is a bug because memory consumption on the previous queue then becomes unbounded, or limited only by the number of struct requests that can be allocated. In other words, it reverts to the same situation we have now as soon as the IO stack has more than one queue. (Just a shorter version of my previous post.) We can solve this by having the bio only point at the queue to which it was originally submitted, since throttling the top level queue automatically throttles all queues lower down the stack. Alternatively the bio can point at the block_device or straight at the backing_dev_info, which is the per-device structure it actually needs to touch. Note! There are two more issues I forgot to mention earlier. 1) One throttle count per submitted bio is too crude a measure. A bio can carry as few as one page or as many as 256 pages. If you take only one throttle count per bio and that data will be transferred over the network then you have to assume that (a little more than) 256 pages of sk_alloc reserve will be needed for every bio, resulting in a grossly over-provisioned reserve. The precise reserve calculation we want to do is per-block device, and you will find hooks like this already living in backing_dev_info. We need to place our own fn+data there to calculate the throttle draw for each bio. Unthrottling gets trickier with variable size throttle draw. In ddsnap, we simply write the amount we drew from the throttle into (the private data of) bio for use later by unthrottle, thus avoiding the issue that the bio fields we used to calculate might have changed during the lifetime of the bio. This would translate into one more per-bio field. 2) Exposing the per-block device throttle limits via sysfs or similar is really not a good long term solution for system administration. Imagine our help text: "just keep trying smaller numbers until your system deadlocks". We really need to figure this out internally and get it correct. I can see putting in a temporary userspace interface just for experimentation, to help determine what really is safe, and what size the numbers should be to approach optimal throughput in a fully loaded memory state. Regards, Daniel Regards, Daniel - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
