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

[Avi Kivity <avi@redhat.com>]

On 04/25/2010 06:29 PM, Dan Magenheimer wrote:
>>> While I admit that I started this whole discussion by implying
>>> that frontswap (and cleancache) might be useful for SSDs, I think
>>> we are going far astray here.  Frontswap is synchronous for a
>>> reason: It uses real RAM, but RAM that is not directly addressable
>>> by a (guest) kernel.  SSD's (at least today) are still I/O devices;
>>> even though they may be very fast, they still live on a PCI (or
>>> slower) bus and use DMA.  Frontswap is not intended for use with
>>> I/O devices.
>>>
>>> Today's memory technologies are either RAM that can be addressed
>>> by the kernel, or I/O devices that sit on an I/O bus.  The
>>> exotic memories that I am referring to may be a hybrid:
>>> memory that is fast enough to live on a QPI/hypertransport,
>>> but slow enough that you wouldn't want to randomly mix and
>>> hand out to userland apps some pages from "exotic RAM" and some
>>> pages from "normal RAM".  Such memory makes no sense today
>>> because OS's wouldn't know what to do with it.  But it MAY
>>> make sense with frontswap (and cleancache).
>>>
>>> Nevertheless, frontswap works great today with a bare-metal
>>> hypervisor.  I think it stands on its own merits, regardless
>>> of one's vision of future SSD/memory technologies.
>>>        
>> Even when frontswapping to RAM on a bare metal hypervisor it makes
>> sense
>> to use an async API, in case you have a DMA engine on board.
>>      
> When pages are 2MB, this may be true.  When pages are 4KB and
> copied individually, it may take longer to program a DMA engine
> than to just copy 4KB.
>    

Of course, you have to use a batching API, like virtio or Xen's rings, 
to avoid the overhead.

> But in any case, frontswap works fine on all existing machines
> today.  If/when most commodity CPUs have an asynchronous RAM DMA
> engine, an asynchronous API may be appropriate.  Or the existing
> swap API might be appropriate. Or the synchronous frontswap API
> may work fine too.  Speculating further about non-existent
> hardware that might exist in the (possibly far) future is irrelevant
> to the proposed patch, which works today on all existing x86 hardware
> and on shipping software.
>    

dma engines are present on commodity hardware now:

http://en.wikipedia.org/wiki/I/O_Acceleration_Technology

I don't know if consumer machines have them, but servers certainly do.  
modprobe ioatdma.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.