On Mon, 2009-10-12 at 09:26 +0800, Wu Fengguang wrote: > On Sun, Oct 11, 2009 at 07:25:17PM +0800, Peter Zijlstra wrote: > > On Sun, 2009-10-11 at 18:50 +0800, Wu Fengguang wrote: > > > > > > Sorry for the confusion, but I mean, filesystems have to limit > > > nr_writeback (directly or indirectly via the block io queue), > > > otherwise it either hit nr_dirty to 0 (with the loop), or let > > > nr_writeback grow out of control (without the loop). > > > > Doesn't this require the writeback queue to have a limit < dirty_thresh? > > Yes, this is the key (open) issue. For now we have nothing to limit > > nr_writeback < dirty_thresh > > > Or more specifically, for the bdi case: > > > > bdi_dirty + bdi_writeback + bdi_unstable <= bdi_thresh > > > > we require that the writeback queue be smaller than bdi_thresh, which > > could be quite difficult, since bdi_thresh can easily be 0. > > We could apply a MIN_BDI_DIRTY_THRESH. Because the bdi threshold is > estimated from writeback events, so bdi_thresh must be non-zero to > allow some writeback pages in flight :) Not really, suppose you have 1000 NFS clients, of which you only use a hand full at a time. Then the bdi_thresh will be 0 for most of them, and only when you switch to one it'll start growing. But it's perfectly reasonable to expect bdi_thresh=0 to work. It just reverts to sync behaviour, we write out everything and block until they're all gone from writeback state. MIN_BDI_DIRTY_THRESH != 0, will have a side effect of imposing a max number of BDIs on the system, I'm not sure you want to go there. > > Without observing the bdi_thresh constraint we can have: > > > > \Sum_(over bdis) writeback_queue_size > > > > dirty pages outstanding, which could be significantly higher than > > dirty_thresh. > > Yes. Maybe we could do some per-bdi and/or global writeback wait > queue (ie. some generalized version of the patch 20: NFS: introduce > writeback wait queue). > > The per-bdi writeback queue size should ideally be proportional to its > available writeback bandwidth. MIN_BDI_DIRTY_THRESH could be defined > to (2*bdi_writeback_bandwidth) or something close. And if the resulted > bdi limits turn out to be too large for a small memory system, we just > let the global limit kick in. For such small memory systems, it is > very likely there are only one bdi. So it is not likely to lose > fairness to base its limits on available memory instead of device > capability. I'm not seeing why. By simply keeping that loop we're good again, and can have a writeback queue that works well in the saturated case. -- 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
