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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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