Re: [PATCH -v3 00/10] THP swap: Delay splitting THP during swapping out

From: Andi Kleen
Date: Thu Sep 22 2016 - 19:53:07 EST

"Chen, Tim C" <tim.c.chen@xxxxxxxxx> writes:

>>So this is impossible without THP swapin. While 2M swapout makes a lot of
>>sense, I doubt 2M swapin is really useful. What kind of application is 'optimized'
>>to do sequential memory access?

Anything that touches regions larger than 4K and we want to do the
kernel do minimal work to manage the swapping.

> We waste a lot of cpu cycles to re-compact 4K pages back to a large page
> under THP. Swapping it back in as a single large page can avoid
> fragmentation and this overhead.

Also splitting something just to merge it again is wasteful.

A lot of big improvements in the block and VM and network layers
over the years came from avoiding that kind of wasteful work.