Messages in this thread | | | Date | Tue, 07 Oct 2008 17:49:49 +0200 | From | Andrea Righi <> | Subject | Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio |
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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!).
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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