Re: [PATCH 21/23] mm: per device dirty threshold

["John Stoffel" <john@stoffel.org>]

Peter> Scale writeback cache per backing device, proportional to its
Peter> writeout speed.  By decoupling the BDI dirty thresholds a
Peter> number of problems we currently have will go away, namely:

Ah, this clarifies my questions!  Thanks!

Peter>  - mutual interference starvation (for any number of BDIs);
Peter>  - deadlocks with stacked BDIs (loop, FUSE and local NFS mounts).

Peter> It might be that all dirty pages are for a single BDI while
Peter> other BDIs are idling. By giving each BDI a 'fair' share of the
Peter> dirty limit, each one can have dirty pages outstanding and make
Peter> progress.

Question, can you change (shrink) the limit on a BDI while it has IO
in flight?  And what will that do to the system?  I.e. if you have one
device doing IO, so that it has a majority of the dirty limit.  Then
another device starts IO, and it's a *faster* device, how
quickly/slowly does the BDI dirty limits change for both the old and
new device?  

Peter> A global threshold also creates a deadlock for stacked BDIs;
Peter> when A writes to B, and A generates enough dirty pages to get
Peter> throttled, B will never start writeback until the dirty pages
Peter> go away. Again, by giving each BDI its own 'independent' dirty
Peter> limit, this problem is avoided.

Peter> So the problem is to determine how to distribute the total
Peter> dirty limit across the BDIs fairly and efficiently. A DBI that

You mean BDI here, not DBI.  

Peter> has a large dirty limit but does not have any dirty pages
Peter> outstanding is a waste.

Peter> What is done is to keep a floating proportion between the DBIs
Peter> based on writeback completions. This way faster/more active
Peter> devices get a larger share than slower/idle devices.

Does a slower device get a BDI which is calculated to keep it's limit
under a certain number of seconds of outstanding IO?  This way no
device can build up more than say 15 seconds of outstanding IO to
flush at any one time.  

Thanks!
John

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