Re: kern/55166: uvm_pdpolicy_clock params (anon/exec/file max/min)defaults

[Joerg Sonnenberger <joerg%bec.de@localhost>]

The following reply was made to PR kern/55166; it has been noted by GNATS.

From: Joerg Sonnenberger <joerg%bec.de@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: kern-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
	netbsd-bugs%netbsd.org@localhost, tsutsui%ceres.dti.ne.jp@localhost
Subject: Re: kern/55166: uvm_pdpolicy_clock params (anon/exec/file
 max/min)defaults
Date: Mon, 13 Apr 2020 19:55:11 +0200

 On Mon, Apr 13, 2020 at 03:30:04PM +0000, Izumi Tsutsui wrote:
 > The following reply was made to PR kern/55166; it has been noted by GNATS.
 > 
 > From: Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>
 > To: gnats-bugs%netbsd.org@localhost
 > Cc: joerg%bec.de@localhost, tsutsui%ceres.dti.ne.jp@localhost
 > Subject: Re: kern/55166: uvm_pdpolicy_clock params (anon/exec/file max/min)defaults
 > Date: Tue, 14 Apr 2020 00:29:16 +0900
 > 
 >  joerg@ wrote:
 >  
 >  >  > There parameters can be changed by sysctl(8), but initial values
 >  >  > are hardcorded in uvmpdpol_init():
 >  >  
 >  >  What about actually tuning the initial parameters on the amount of RAM
 >  >  in the system? That seems much more sensible than hard-coding different
 >  >  option sets in various kernels. It's not like it is going to add a lot
 >  >  of (cold) code.
 >  
 >  I'm afraid we don't have reasonable fomula to determine optimized
 >  parameters per the amount of RAM.  IIRC, there were several discussion
 >  about the default value ("current filemax was too large" etc.) just
 >  after UBC was introduced, but I didn't see any conclusion.
 >  
 >  My suggestion (making the initial parameters configurable) is just
 >  an intermediate fix.
 
 Sure, we are quite bad at auto-tuning based on avalable memory in 
 general. That doesn't mean we shouldn't try. We don't have to go for 
 perfect here either. Pick a reasonable size for boosting exec 
 specifically should be enough.
 
 >  >  filemax seems questionable to me, the rest sound a useful default for
 >  >  systems with <= 16MB RAM. 
 >  
 >  I guess the default BUFCACHE (vm.bufcache / vm.bufmem, 15% of RAM) is
 >  enough and no extra file cache is necessary for less memory systems.
 
 But the buffer cache is only used for meta-data, not for regular
 caching?
 
 Joerg