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Date: Mon, 16 Apr 2012 12:22:10 -0600 From: Stephan Uphoff <ups@...gle.com> To: Arnd Bergmann <arnd@...db.de> Cc: Minchan Kim <minchan@...nel.org>, linaro-kernel@...ts.linaro.org, android-kernel@...glegroups.com, linux-mm@...ck.org, "Luca Porzio (lporzio)" <lporzio@...ron.com>, Alex Lemberg <alex.lemberg@...disk.com>, linux-kernel@...r.kernel.org, Saugata Das <saugata.das@...aro.org>, Venkatraman S <venkat@...aro.org>, Yejin Moon <yejin.moon@...sung.com>, Hyojin Jeong <syr.jeong@...sung.com>, "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org> Subject: Re: swap on eMMC and other flash 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-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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