On 04/17/2012 06:12 AM, Stephan Uphoff wrote:
Hi Arnd,
On Mon, Apr 16, 2012 at 12:59 PM, Arnd Bergmannarnd@arndb.de 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:
- 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 http://marc.info/?l=linux-mm&m=133462346928786&w=2