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

[Nitin Gupta]

On 04/25/2010 05:46 PM, Avi Kivity wrote:
> 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).
> 

No. We do not want to issue discard for every page as soon as it is freed.
I'm not flash expert but I guess issuing erase is just too expensive to be
issued so frequently. OTOH, ramzswap needs a callback for every page and as
soon as it is freed.


>> 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.
> 

Ok, but still all this bio allocation and block layer overhead seems
unnecessary and is easily avoidable. I think frontswap code needs
clean up but at least it avoids all this bio overhead.

>>   - 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?
> 

frontswap simply calls frontswap_flush_page() in swap_entry_free() i.e. as
soon as a swap slot is freed. No bio allocation etc.

>>> 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.
> 

Optimizing swap-to-flash is surely desirable but this problem is separate
from ramzswap or frontswap optimization. For the latter, I think dealing
with bio's, going through block layer is plain overhead.

>> 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.
>

Maybe incremental development is better? Stabilize and refine existing
code and gradually move to async API, if required in future?

Thanks,
Nitin

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