Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio

[Andrea Righi <righi.andrea@gmail.com>]

Michael Rubin wrote:
> On Fri, Sep 12, 2008 at 1:18 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
>> One thing to think about please: Michael Rubin is hitting problems with
>> the existing /proc/sys/vm/dirty-ratio.  Its present granularity of 1%
>> is just too coarse for really large machines, and as
>> memory-size/disk-speed ratios continue to increase, this will just get
>> worse.
> 
> Re-sending since I top-posted before. Never again. Also adding more
> thoughts on a byte based interface.
> 
> Currently the problem we are hitting is that we cannot specify pdflush
> to have background limits less than 1% of memory. I am currently
> finishing up a patch right now that adds a dirty_ratio_millis
> interface.  I hope to submit the patch to LKML by the end of the week.
> 
> The idea is that we don't want to break backwards compatibility and we
> also don't want to have two conflicting knobs in the sysctl or
> /proc/sys/vm/ space. I thought adding a new knob for those who want to
> specify finer grained functionality was a compromise. So the patch has
> a vm_dirty_ratio and a vm_dirty_ratio_millis interface. The first to
> specify 0-100% and the second to specify .0 to .999%.
> 
> So to represent 0.125% of RAM we set
> vm_dirty_ratio = 0
> vm_dirty_ratio_millis = 125
> 
> The same for the background_ratio.
> 
> I would also prefer using a bytes interface but I am not sure how to
> offer that without  either removing the legacy interface of the ratios
> or by offering a concurrent interface that might be confusing such as
> when users are looking at the old one and not aware of a new one.
> 
> Any feedback?
> 
> mrubin

I think using millis is ok today, but it may not scale well to systems
with 1TB of memory (in this case the min granularity would be 10MB).

A bytes/pages interface would resolve such problem also for tomorrow
machines.

Moreover, wouldn't it be safer to set them mutually exclusive? I mean,
writing a value != 0 to vm_dirty_millis automatically sets
vm_dirty_ratio to 0 (disabled) and vice versa (this could be implemented
using an appropriate .proc_handler for example).

OK, I would like to set percentages like 12.456%, but if we don't do so
a simple "sysctl -p" could create unexpected behaviours, reconfiguring
the vm_dirty_ratio and not vm_dirty_ratio_millis for example.

The same should be valid also for a bytes/pages interface, so setting
vm_dirty_bytes != 0 (or vm_dirty_pages) should "disable" vm_dirty_ratio
and vice versa.

Thanks,
-Andrea