Re: [PATCH - RFC] allow setting vm_dirty below 1% for large memory machines

[Andrew Morton <akpm@osdl.org>]

On Wed, 10 Jan 2007 14:29:35 +1100
Neil Brown <neilb@suse.de> wrote:

> > 
> > It would be better if we can avoid creating the second global variable.  Is
> > it not possible to remove dirty_ratio?  Make everything work off
> > vm_dirty_kb and do arithmetricks at the /proc/sys/vm/dirty_ratio interface?
> 
> Uhmmm... not sure what you are thinking.
> I guess we could teach vm.dirty_ratio to take a floating point number
> (but does sysctl understand that?) so we could set it to 0.01 or
> similar, but that is missing the point in a way.  We don't really want
> to set a small ratio.  We want to set a small maximum number.

I mean remove the kernel-internal dirty_ratio variable and use
/proc/sys/vm/dirty_ratio as an accessor to `long vm_dirty_kb', with
appropriate conversions when /proc/sys/vm/dirty_ratio is written to and
read from.

> It could make lots of sense to have two numbers.  A ratio that wins on
> a small memory machine and a fixed number that wins on a large memory
> machine.  Different trade offs are more significant in the different
> cases.

hm.

> > 
> > We should perform the same conversion to dirty_background_ratio, I suspect.
> > 
> 
> I didn't add a fixed limit for dirty_background_ratio as it seemed
> reasonable to assume that (dirty_background_ratio / dirty_ratio) was a
> meaningful value, and just multiplied the final 'dirty' figure by this
> ration to get the 'background' figure.

Sounds complex.  Better, I think, to create (and recommend) vm_dirty_kb and
vm_dirty_background_kb and deprecate the old knobs.

> > And these guys should be `long', not `int'.  Otherwise things will go
> > pearshaped at 2 tabbybytes.
> 
> I don't think so.  You would need to have blindingly fast storage
> before there would be any interest in vm_dirty_kb getting anything
> close to t*bytes.  But I guess we can make it 'unsigned long' if it
> helps.
> 

A 16TB machine would overflow that int by default.