On Sat, 2007-10-27 at 16:02 -0500, Steve French wrote: > On 10/27/07, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote: > > Hi, > > > > I had me a little look at bdi usage in networked filesystems. > > > > NFS, CIFS, (smbfs), AFS, CODA and NCP > > > > And of those, NFS is the only one that I could find that creates > > backing_dev_info structures. The rest seems to fall back to > > default_backing_dev_info. > > > > With my recent per bdi dirty limit patches the bdi has become more > > important than it has been in the past. While falling back to the > > default_backing_dev_info isn't wrong per-se, it isn't right either. > > > > Could I implore the various maintainers to look into this issue for > > their respective filesystem. I'll try and come up with some patches to > > address this, but feel free to beat me to it. > > I would like to understand more about your patches to see what bdi > values makes sense for CIFS and how to report possible congestion back > to the page manager. So, what my recent patches do is carve up the total writeback cache size, or dirty page limit as we call it, proportionally to a BDIs writeout speed. So a fast device gets more than a slow device, but will not starve it. However, for this to work, each device, or remote backing store in the case of networked filesystems, need to have a BDI. > I had been thinking about setting bdi->ra_pages > so that we do more sensible readahead and writebehind - better > matching what is possible over the network and what the server > prefers. Well, you'd first have to create backing_dev_info instances before setting that value :-) > SMB/CIFS Servers typically allow a maximum of 50 requests > in parallel at one time from one client (although this is adjustable > for some). That seems like a perfect point to set congestion. So in short, stick a struct backing_dev_info into whatever represents a client, initialize it using bdi_init(), destroy using bdi_destroy(). Mark it congested once you have 50 (or more) outstanding requests, clear congestion when you drop below 50. and you should be set.
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