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

[Andrea Righi]

Balbir Singh wrote:
> Michael Rubin wrote:
>> On Fri, Sep 12, 2008 at 1:18 PM, Andrew Morton
>> <akpm at 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.
>>
> 
> Just provide a vm_dirty_ration_in_bytes interface and keep it in sync with
> vm_dirty_ratio (they are just two representations of the same internal value)
> and for higher resolution propose that users use the bytes interface.

Hi Balbir,

now that I read carefully the documentation, the description in
Documentation/filesystems/proc.txt seems to be a bit misleading. In
proc.txt we say that dirty_ratio and dirty_background_ratio are "a
percentage of total system memory", but in mm/page-writeback.c we apply
the percentages to the dirtyable memory: free pages + reclaimable pages.
So, first of all I think we should clarify this in the documentation...

Saying that, keeping in sync the vm_dirty_amount_in_bytes according to
dirty_ratio_in_percentage is not a trivial task. One is a static value,
the other depends on the dirtyable memory in the system. If we want to
preserve the same behaviour we should do the following:

dirty_ratio = x => dirty_amount_in_bytes = x * dirtyable_memory / 100

dirty_amount_in_bytes = y => dirty_ratio = y / dirtyable_memory * 100

But anytime the dirtyable memory (or the total memory in the system)
changes we should update both values accordingly to preserve the
coherency between them (ouch!).

Possible solutions:

1) introduce fine-grained dirty_ratio handling decimals by an opportune
   parser (disadvantage: this would break the compatibility with all the
   userspace apps that expect to read an int from vm_dirty_ratio)

2) introduce dirty_ratio + dirty_ratio_millis (disadvantage: can
   generate unexpected behaviours when something is written to
   dirty_ratio ignoring the existence of dirty_ratio_millis)

3) introduce dirty_ratio + dirty_amount_in_bytes mutually exclusive,
   writing to one automatically "disable" the other (disadvantage:
   writing to dirty_ratio ignoring dirty_amount_in_bytes can cause
   unexpected behaviours)

4) introduce dirty_ratio + dirty_amount_in_bytes and change the
   old behaviour: when something is written to dirty_ratio,
   dirty_amount_in_bytes is evaluated in function of totalram_pages (or
   the memcg limit) and then we always use this static value, instead of
   something that depends on the dirtyable memory - we can easily update
   dirty_amount_in_bytes also when totalram_pages or the memcg limit
   changes (disadvantage: change an old - working - behaviour).

5) handle fine-grained dirty_ratio decimals by an opportune parser when
   writing something to dirty_ratio; export the percentage units via
   dirty_ratio, and the decimals via dirty_ratio_decimals; writing to
   dirty_ratio_decimals is not allowed.

I tend to choose 5. The same for dirty_background_ratio.

-Andrea
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