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Date: Tue, 8 Mar 2016 21:07:46 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Scott Long <scottl@samsco.org>
Cc: freebsd-stable@freebsd.org
Subject: Re: kernel: mps0: Out of chain frames, consider increasing
 hw.mps.max_chains.
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On Mon, Mar 07, 2016 at 02:10:12PM +0300, Slawa Olhovchenkov wrote:

> > >>> This allocated one for all controllers, or allocated for every controller?
> > >> 
> > >> It’s per-controller.
> > >> 
> > >> I’ve thought about making the tuning be dynamic at runtime.  I
> > >> implemented similar dynamic tuning for other drivers, but it seemed
> > >> overly complex for low benefit.  Implementing it for this driver
> > >> would be possible but require some significant code changes.
> > > 
> > > What cause of chain_free+io_cmds_active << max_chains?
> > > One cmd can use many chains?
> > 
> > Yes.  A request uses and active command, and depending on the size of the I/O,
> > it might use several chain frames.

I am play with max_chains and like significant cost of handling
max_chains: with 8192 system resonded badly vs 2048. Now try 3192,
response like with 2048.