Re: [patch 4/5] mm: writeback: throttle __GFP_WRITE on per-zone dirty limits

[Minchan Kim]

Hi Andi,

On Tue, Jul 26, 2011 at 5:37 AM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>> The global dirty limits are put in proportion to the respective zone's
>> amount of dirtyable memory and the allocation denied when the limit of
>> that zone is reached.
>>
>> Before the allocation fails, the allocator slowpath has a stage before
>> compaction and reclaim, where the flusher threads are kicked and the
>> allocator ultimately has to wait for writeback if still none of the
>> zones has become eligible for allocation again in the meantime.
>>
>
> I don't really like this. It seems wrong to make memory
> placement depend on dirtyness.
>
> Just try to explain it to some system administrator or tuner: her
> head will explode and for good reasons.
>
> On the other hand I like doing round-robin in filemap by default
> (I think that is what your patch essentially does)
> We should have made Âthis default long ago. It avoids most of the
> "IO fills up local node" problems people run into all the time.
>
> So I would rather just change the default in filemap allocation.
>
> That's also easy to explain.

Just out of curiosity.
Why do you want to consider only filemap allocation, not IO(ie,
filemap + sys_[read/write]) allocation?

-- 
Kind regards,
Minchan Kim
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