Re: [Qemu-devel] [PATCH 4/4] Reallocate dma buffers in read/write path if needed

[Anthony Liguori]

Ryan Harper wrote:
> I'd rather avoid any additional accounting overhead of a pool.  If 4MB
> is a reasonable limit, lets make that the new max.  I can do some
> testing to see where we drop off on performance improvements.  We'd
> have a default buffer size (smaller than the previous 64, and now 128k
> buf size) that is used when we allocate scsi requests; scanning through
> send_command() provides a good idea of other scsi command buf usage; and
> on reads and writes, keep the capping logic we've had all along, but
> bump the max size up to something like 4MB -- or whatever tests results
> show as being ideal. 
>
> In all, it seems silly to worry about this sort of thing since the
> entire process could be contained with process ulimits if this is really
> a concern.  Are we any more concerned that by splitting the requests
> into many smaller requests that we're wasting cpu, pegging the
> processor to 100% in some cases?
>   

There are two concerns with allowing the guest to alloc arbitrary 
amounts of memory.  The first is that QEMU is not written in such a way 
to be robust in the face of out-of-memory conditions so if we run out of 
VA space, an important malloc could fail and we'd fall over.

The second concern is that if a guest can allocate arbitrary amounts of 
memory, it could generate a swap storm.  Unfortunately, AFAIK, Linux is 
not yet to a point where it can deal with swap fairness.  Hopefully this 
is a limitation that the IO controller folks are taking into account.

Regards,

Anthony Liguori