Messages in this thread |  | | Subject | Re: dirty balancing deadlock | From | Miklos Szeredi <> | Date | Tue, 20 Feb 2007 09:47:11 +0100 |
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> > How about this? > > > > Solves the FUSE deadlock, but not the throttle_vm_writeout() one. > > I'll try to tackle that one as well. > > > > If the per-bdi dirty counter goes below 16, balance_dirty_pages() > > returns. > > > > Does the constant need to tunable? If it's too large, then the global > > threshold is more easily exceeded. If it's too small, then in a tight > > situation progress will be slower. > > Ok, what is supposed to happen here is that filesystems are supposed to > be throttled from making more dirty pages when the system is over the > threshold. Even if filesystem A doesn't have much to contribute, and > filesystem B is the cause of 99% of the dirty pages, the goal of the > threshold is to prevent more dirty data from happening, and filesystem A > should block.
Which is the cause of the current deadlock. But if we allow filesystem A to go into the red just a little, the deadlock is avoided, because it can continue to make progress with cleaning the dirtyness produced by B.
The maximum that filesystems can go over the limit will be
(16 + epsilon) * number-of-queues
This is usually insignificant compared to the limit itself (~2000 pages on a machine with 32MB)
However with thousands of fuse mounts this may become a problem, as each filesystem gets a separate queue. In theory, just 2 pages are enough to always make progress, but current dirty balancing can't enforce this, as the ratelimit is at least 8 pages.
So there may have to be some more strict page accounting within fuse itself, but that doesn't change the overall concept I think.
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