Re: [RFC] mm: remove swapcache page early

[Seth Jennings]

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!

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

> 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?

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.

Thanks,
Seth

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