Re: [PATCH v4 0/9] mm/swap: Regular page swap optimizations

[Huang\, Ying]

Hi, Minchan,

Minchan Kim <minchan@xxxxxxxxxx> writes:

> Hi Jan,
>
> On Mon, Jan 02, 2017 at 04:48:41PM +0100, Jan Kara wrote:
>> Hi,
>> 
>> On Tue 27-12-16 16:45:03, Minchan Kim wrote:
>> > > Patch 3 splits the swap cache radix tree into 64MB chunks, reducing
>> > >         the rate that we have to contende for the radix tree.
>> > 
>> > To me, it's rather hacky. I think it might be common problem for page cache
>> > so can we think another generalized way like range_lock? Ccing Jan.
>> 
>> I agree on the hackyness of the patch and that page cache would suffer with
>> the same contention (although the files are usually smaller than swap so it
>> would not be that visible I guess). But I don't see how range lock would
>> help here - we need to serialize modifications of the tree structure itself
>> and that is difficult to achieve with the range lock. So what you would
>> need is either a different data structure for tracking swap cache entries
>> or a finer grained locking of the radix tree.
>
> Thanks for the comment, Jan.
>
> I think there are more general options. One is to shrink batching pages like
> Mel and Tim had approached.
>
> https://patchwork.kernel.org/patch/9008421/
> https://patchwork.kernel.org/patch/9322793/

This helps to reduce the lock contention on radix tree of swap cache.
But splitting swap cache has much better performance.  So we switched
from that solution to current solution.

> Or concurrent page cache by peter.
>
> https://www.kernel.org/doc/ols/2007/ols2007v2-pages-311-318.pdf

I think this is good, it helps swap and file cache.  But I don't know
whether other people want to go this way and how much effort will be
needed.

In contrast, splitting swap cache is quite simple, for implementation
and review.  And the effect is good.

Best Regards,
Huang, Ying

> Ccing Nick who might have an interest on lockless page cache.
>
> Thanks.