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

[Avi Kivity <avi@redhat.com>]

On 04/25/2010 06:11 AM, Nitin Gupta wrote:
> 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).
>    

Doesn't flash have similar requirements?  The earlier you discard, the 
likelier you are to reuse an erase block (or reduce the amount of copying).

> 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.
>    

That's fine, swap works under low memory conditions by using reserves.

>   - 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).
>    

How does frontswap do this?  Does it maintain its own data structures?

>> 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.
>    

I'm not sure.  Swap-to-flash will soon be everywhere.   If it's slow, 
people will feel it a lot more than ramzswap slowness.

> 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.
>    

Even for copying to RAM an async API is wanted, so you can dma it 
instead of copying.

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