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

[Huang\, Ying]

Rik van Riel <riel@xxxxxxxxxx> writes:

> On Thu, 2016-09-22 at 15:56 -0700, Shaohua Li wrote:
>> On Wed, Sep 07, 2016 at 09:45:59AM -0700, Huang, Ying wrote:
>> >Â
>> > - It will help the memory fragmentation, especially when the THP is
>> > Â heavily used by the applications.ÂÂThe 2M continuous pages will
>> > be
>> > Â free up after THP swapping out.
>> 
>> 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?
>
> I suspect a lot of this will depend on the ratio of storage
> speed to CPU & RAM speed.
>
> When swapping to a spinning disk, it makes sense to avoid
> extra memory use on swapin, and work in 4kB blocks.

For spinning disk, the THP swap optimization will be turned off in
current implementation.  Because huge swap cluster allocation based on
swap cluster management, which is available only for non-rotating block
devices (blk_queue_nonrot()).

> When swapping to NVRAM, it makes sense to use 2MB blocks,
> because that storage can handle data faster than we can
> manage 4kB pages in the VM.

Best Regards,
Huang, Ying