Messages in this thread | | | Date | Wed, 8 Oct 2008 10:16:42 +0900 | From | KAMEZAWA Hiroyuki <> | Subject | Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio |
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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!). > > 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 ==
Thanks, -Kame
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