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    SubjectRe: 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.


    Kind regards,
    Minchan Kim

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