Messages in this thread | | | Date | Wed, 08 Oct 2008 18:43:57 +0530 | From | Balbir Singh <> | Subject | Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio |
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KAMEZAWA Hiroyuki wrote: > On Tue, 07 Oct 2008 17:49:49 +0200 > Andrea Righi <righi.andrea@gmail.com> wrote: > >> Balbir Singh wrote: >>> 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. >>>> >>> 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!). >>
I see what you mean.
>> 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. >> > > Hmm... I agree to "5"... like this ? > == > prvoides > - vm.dirty_ratio (1/100) > - vm.dirty_ratio_percentmille(1/100,000, pcm) > > and allow > #echo 0.05 > vm/dirty_ratio > #cat vm/dirty_ratio > 0 > #cat vm/dirty_ratio_percentmille > 500 > ==
I guess this would be the easiest way forward, I'll let you select the granularity of the interface and its meaning.
-- Balbir
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