Re: swap on eMMC and other flash

From: Minchan Kim
Date: Mon Apr 16 2012 - 22:18:53 EST

On 04/17/2012 06:12 AM, Stephan Uphoff wrote:
Hi Arnd,

On Mon, Apr 16, 2012 at 12:59 PM, Arnd Bergmann<arnd@xxxxxxxx> wrote:
On Monday 16 April 2012, Stephan Uphoff wrote:
opportunity to plant a few ideas.

In contrast to rotational disks read/write operation overhead and
costs are not symmetric.
While random reads are much faster on flash - the number of write
operations is limited by wearout and garbage collection overhead.
To further improve swapping on eMMC or similar flash media I believe
that the following issues need to be addressed:

1) Limit average write bandwidth to eMMC to a configurable level to
guarantee a minimum device lifetime
2) Aim for a low write amplification factor to maximize useable write bandwidth
3) Strongly favor read over write operations

Lowering write amplification (2) has been discussed in this email
thread - and the only observation I would like to add is that
over-provisioning the internal swap space compared to the exported
swap space significantly can guarantee a lower write amplification
factor with the indirection and GC techniques discussed.

Yes, good point.

I believe the swap functionality is currently optimized for storage
media where read and write costs are nearly identical.
As this is not the case on flash I propose splitting the anonymous
inactive queue (at least conceptually) - keeping clean anonymous pages
with swap slots on a separate queue as the cost of swapping them
out/in is only an inexpensive read operation. A variable similar to
swapiness (or a more dynamic algorithmn) could determine the
preference for swapping out clean pages or dirty pages. ( A similar
argument could be made for splitting up the file inactive queue )

I'm not sure I understand yet how this would be different from swappiness.

As I see it swappiness determines the ratio for paging out file backed
as compared to anonymous, swap backed pages.
I would like to further be able to set the ratio for throwing away
clean anonymous pages with swap slots ( that are easy to read back in)
as compared to writing out dirty anonymous pages to swap.

We can apply the rule in file-lru list too and we already have ISOLATE_CLEAN mode to select victim pages in LRU list so it should work.

For selecting clean anon pages with swap slot, we need more looking.
Recent, Dan had a question about it and Hugh answered it.
Look at the

Kind regards,
Minchan Kim
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