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Date: Tue, 17 Apr 2012 11:05:06 +0900 From: Minchan Kim <minchan@...nel.org> To: Arnd Bergmann <arnd@...db.de> CC: Stephan Uphoff <ups@...gle.com>, 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 Hi Arnd, On 04/17/2012 03:59 AM, Arnd Bergmann 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. > >> 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. > > While swap is just a special case for anonymous memory in writeback > rather than file backed pages, I think what you want here is a tuning > knob that decides whether we should discard a clean page or write back > a dirty page under memory pressure. I have to say that I don't know > whether we already have such a knob or whether we already treat them > differently, but it is certainly a valid observation that on hard > drives, discarding a clean page that is likely going to be needed > again has about the same overhead as writing back a dirty page > (i.e. one seek operation), while on flash the former would be much > cheaper than the latter. It seems to make sense with considering asymmetric of flash and there is a CFLRU(Clean First LRU)[1] paper about it. You might already know it. Anyway if you don't aware of it, I hope it helps you. [1] http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCkQFjAA&url=http%3A%2F%2Fstaff.ustc.edu.cn%2F~jpq%2Fpaper%2Fflash%2F2006-CASES-CFLRU-%2520A%2520Replacement%2520Algorithm%2520for%2520Flash%2520Memory.pdf&ei=G8-MT5jGIqnj0gGMzJyCCg&usg=AFQjCNHybc5rUvuAlMylOUNwsHoFmWegzw&sig2=Uu5LDD3suso0QHsfD7yZ9Q -- Kind regards, Minchan Kim -- 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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