Re: [patch 5/5] mm: refault distance-based file cache sizing

[Minchan Kim <minchan@kernel.org>]

On 05/02/2012 12:38 AM, Johannes Weiner wrote:

> On Tue, May 01, 2012 at 11:13:30PM +0900, Minchan Kim wrote:
>> Hi Hannes,
>>
>> On Tue, May 01, 2012 at 10:41:53AM +0200, Johannes Weiner wrote:
>>> To protect frequently used page cache (workingset) from bursts of less
>>> frequently used or one-shot cache, page cache pages are managed on two
>>> linked lists.  The inactive list is where all cache starts out on
>>> fault and ends on reclaim.  Pages that get accessed another time while
>>> on the inactive list get promoted to the active list to protect them
>>> from reclaim.
>>>
>>> Right now we have two main problems.
>>>
>>> One stems from numa allocation decisions and how the page allocator
>>> and kswapd interact.  The both of them can enter into a perfect loop
>>> where kswapd reclaims from the preferred zone of a task, allowing the
>>> task to continuously allocate from that zone.  Or, the node distance
>>> can lead to the allocator to do direct zone reclaim to stay in the
>>> preferred zone.  This may be good for locality, but the task has only
>>
>> Understood.
>>
>>> the inactive space of that one zone to get its memory activated.
>>> Forcing the allocator to spread out to lower zones in the right
>>> situation makes the difference between continuous IO to serve the
>>> workingset, or taking the numa cost but serving fully from memory.
>>
>> It's hard to parse your word due to my dumb brain.
>> Could you elaborate on it?
>> It would be a good if you say with example.
> 
> Say your Normal zone is 4G (DMA32 also 4G) and you have 2G of active
> file pages in Normal and DMA32 is full of other stuff.  Now you access
> a new 6G file repeatedly.  First it allocates from Normal (preferred),
> then tries DMA32 (full), wakes up kswapd and retries all zones.  If
> kswapd then frees pages at roughly the same pace as the allocator
> allocates from Normal, kswapd never goes to sleep and evicts pages
> from the 6G file before they can get accessed a second time.  Even
> though the 6G file could fit in memory (4G Normal + 4G DMA32), the
> allocator only uses the 4G Normal zone.
> 
> Same applies if you have a load that would fit in the memory of two
> nodes but the node distance leads the allocator to do zone_reclaim()
> and forcing the pages to stay in one node, again preventing the load
> from being fully cached in memory, which is much more expensive than
> the foreign node cost.
> 
>>> up to half of memory, and don't recognize workingset changes that are
>>> bigger than half of memory.
>>
>> Workingset change?
>> You mean if new workingset is bigger than half of memory and it's like
>> stream before retouch, we could cache only part of working set because 
>> head pages on working set would be discared by tail pages of working set
>> in inactive list?
> 
> Spot-on.  I called that 'tail-chasing' in my notes :-) When you are in
> a perpetual loop of evicting pages you will need in a couple hundred
> page faults.  Those couple hundred page faults are the refault
> distance and my code is able to detect these loops and increases the
> space available to the inactive list to end them, if possible.
> 


Thanks! It would be better to add above explanation in cover-letter.


> This is the whole principle of the series.
> 
> If such a loop is recognized in a single zone, the allocator goes for
> lower zones to increase the inactive space.  If such a loop is
> recognized over all allowed zones in the zonelist, the active lists
> are shrunk to increase the inactive space.

>

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-- 
Kind regards,
Minchan Kim

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