Re: [RFC] mm: remove swapcache page early

Minchan Kim
2013-03-28T01:36:42Z

Hi Seth,

On Wed, Mar 27, 2013 at 12:19:11PM -0500, Seth Jennings wrote:
> On 03/26/2013 09:22 PM, Minchan Kim wrote:
> > Swap subsystem does lazy swap slot free with expecting the page
> > would be swapped out again so we can't avoid unnecessary write.
> >
> > But the problem in in-memory swap is that it consumes memory space
> > until vm_swap_full(ie, used half of all of swap device) condition
> > meet. It could be bad if we use multiple swap device, small in-memory swap
> > and big storage swap or in-memory swap alone.
> >
> > This patch changes vm_swap_full logic slightly so it could free
> > swap slot early if the backed device is really fast.
>
> Great idea!

Thanks!

>
> > For it, I used SWP_SOLIDSTATE but It might be controversial.
>
> The comment for SWP_SOLIDSTATE is that "blkdev seeks are cheap". Just
> because seeks are cheap doesn't mean the read itself is also cheap.

The "read" isn't not concern but "write".

> For example, QUEUE_FLAG_NONROT is set for mmc devices, but some of
> them can be pretty slow.

Yeb.

>
> > So let's add Ccing Shaohua and Hugh.
> > If it's a problem for SSD, I'd like to create new type SWP_INMEMORY
> > or something for z* family.
>
> Afaict, setting SWP_SOLIDSTATE depends on characteristics of the
> underlying block device (i.e. blk_queue_nonrot()). zram is a block
> device but zcache and zswap are not.
>
> Any idea by what criteria SWP_INMEMORY would be set?

Just in-memory swap, zram, zswap and zcache at the moment. :)

>
> Also, frontswap backends (zcache and zswap) are a caching layer on top
> of the real swap device, which might actually be rotating media. So
> you have the issue of to different characteristics, in-memory caching
> on top of rotation media, present in a single swap device.

Please read my patch completely. I already pointed out the problem and
Hugh and Dan are suggesting ideas.

Thanks!

>
> Thanks,
> Seth
>
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--
Kind regards,
Minchan Kim