Re: [PATCH 00/23] per device dirty throttling -v10

[John Stoffel]

Peter> Per device dirty throttling patches These patches aim to
Peter> improve balance_dirty_pages() and directly address three
Peter> issues:

Peter>   1) inter device starvation
Peter>   2) stacked device deadlocks
Peter>   3) inter process starvation

Peter> 1 and 2 are a direct result from removing the global dirty
Peter> limit and using per device dirty limits. By giving each device
Peter> its own dirty limit is will no longer starve another device,
Peter> and the cyclic dependancy on the dirty limit is broken.

Ye haa!  This should be a big improvement.  

Peter> In order to efficiently distribute the dirty limit across the
Peter> independant devices a floating proportion is used, this will
Peter> allocate a share of the total limit proportional to the
Peter> device's recent activity.

I'm not sure I like or agree with this.  Shouldn't we be limiting
based on the device's capability to sustain traffic?  So if I have a
RAID device which can read/write a total of 100Mb/sec, while at the
same time I've got a CF device which can do 5Mb/sec, shouldn't we be
more strongly limiting the CF device, even if it is the only device
being written to?  

Of course, I haven't read the patches yet, nor am I qualified to
comment on them in any meanginful way I think.  Hopefully I'm just
missing something key here in the explanation.

Peter> 3 is done by also scaling the dirty limit proportional to the
Peter> current task's recent dirty rate.

Do you mean task or device here?  I'm just wondering how well this
works with a bunch of devices with wildly varying speeds.  

John
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