Re: [RFC, PATCH 00/19] Numa aware LRU lists and shrinkers

[Glauber Costa]

On 01/22/2013 03:21 AM, Dave Chinner wrote:
> On Mon, Jan 21, 2013 at 08:08:53PM +0400, Glauber Costa wrote:
>> On 11/28/2012 03:14 AM, Dave Chinner wrote:
>>> [PATCH 09/19] list_lru: per-node list infrastructure
>>>
>>> This makes the generic LRU list much more scalable by changing it to
>>> a {list,lock,count} tuple per node. There are no external API
>>> changes to this changeover, so is transparent to current users.
>>>
>>> [PATCH 10/19] shrinker: add node awareness
>>> [PATCH 11/19] fs: convert inode and dentry shrinking to be node
>>>
>>> Adds a nodemask to the struct shrink_control for callers of
>>> shrink_slab to set appropriately for their reclaim context. This
>>> nodemask is then passed by the inode and dentry cache reclaim code
>>> to the generic LRU list code to implement node aware shrinking.
>>
>> I have a follow up question that popped up from a discussion between me
>> and my very American friend Johnny Wheeler, also known as Johannes
>> Weiner (CC'd). I actually remember we discussing this, but don't fully
>> remember the outcome. And since I can't find it anywhere, it must have
>> been in a media other than e-mail. So I thought it would do no harm in
>> at least documenting it...
>>
>> Why are we doing this per-node, instead of per-zone?
>>
>> It seems to me that the goal is to collapse all zones of a node into a
>> single list, but since the number of zones is not terribly larger than
>> the number of nodes, and zones is where the pressure comes from, what do
>> we really gain from this?
> 
> The number is quite a bit higher - there are platforms with 5 zones
> to a node. The reality is, though, for most platforms slab
> allocations come from a single zone - they never come from ZONE_DMA,
> ZONE_HIGHMEM or ZONE_MOVEABLE, so there is there is no good reason
> for having cache LRUs for these zones. So, two zones at most.
> 
Yes, but one would expect that most of those special zones would be
present only in the first node, no? (correct me if I am wrong here).

Over that, things should be pretty much addressable.

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