Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

[Nitin Gupta]

On 04/24/2010 11:57 PM, Avi Kivity wrote:
> On 04/24/2010 04:49 AM, Nitin Gupta wrote:
>>
>>> I see.  So why not implement this as an ordinary swap device, with a
>>> higher priority than the disk device?  this way we reuse an API and keep
>>> things asynchronous, instead of introducing a special purpose API.
>>>
>>>      
>> ramzswap is exactly this: an ordinary swap device which stores every page
>> in (compressed) memory and its enabled as highest priority swap.
>> Currently,
>> it stores these compressed chunks in guest memory itself but it is not
>> very
>> difficult to send these chunks out to host/hypervisor using virtio.
>>
>> However, it suffers from unnecessary block I/O layer overhead and
>> requires
>> weird hooks in swap code, say to get notification when a swap slot is
>> freed.
>>    
> 
> Isn't that TRIM?

No: trim or discard is not useful. The problem is that we require a callback
_as soon as_ a page (swap slot) is freed. Otherwise, stale data quickly accumulates
in memory defeating the whole purpose of in-memory compressed swap devices (like ramzswap).

Increasing the frequency of discards is also not an option:
 - Creating discard bio requests themselves need memory and these swap devices
come into picture only under low memory conditions.
 - We need to regularly scan swap_map to issue these discards. Increasing discard
frequency also means more frequent scanning (which will still not be fast enough
for ramzswap needs).

> 
>> OTOH frontswap approach gets rid of any such artifacts and overheads.
>> (ramzswap: http://code.google.com/p/compcache/)
>>    
> 
> Maybe we should optimize these overheads instead.  Swap used to always
> be to slow devices, but swap-to-flash has the potential to make swap act
> like an extension of RAM.
> 

Spending lot of effort optimizing an overhead which can be completely avoided
is probably not worth it.

Also, I think the choice of a synchronous style API for frontswap and cleancache
is justified as they want to send pages to host *RAM*. If you want to use other
devices like SSDs, then these should be just added as another swap device as
we do currently -- these should not be used as frontswap storage directly.

Thanks,
Nitin
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