I really like where this is going and would like to use the 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. 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 ) The problem of limiting the average write bandwidth reminds me of enforcing cpu utilization limits on interactive workloads. Just as with cpu workloads - using the resources to the limit produces poor interactivity. When interactivity suffers too much I believe the only sane response for an interactive device is to limit usage of the swap device and transition into a low memory situation - and if needed - either allowing userspace to reduce memory usage or invoking the OOM killer. As a result low memory situations could not only be encountered on new memory allocations but also on workload changes that increase the number of dirty pages. A wild idea to avoid some writes altogether is to see if de-duplication techniques can be used to (partially?) match pages previously written so swap. In case of unencrypted swap (or encrypted swap with a static key) swap pages on eMMC could even be re-used across multiple reboots. A simple version would just compare dirty pages with data in their swap slots as I suspect (but really don't know) that some user space algorithms (garbage collection?) dirty a page just temporarily - eventually reverting it to the previous content. Stephan -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
