Re: [Qemu-devel] [RFC 1/2] pci-dma-api-v1

[Anthony Liguori <anthony@codemonkey.ws>]

Blue Swirl wrote:
> On 11/28/08, Andrea Arcangeli <aarcange@redhat.com> wrote:
>   
>> On Thu, Nov 27, 2008 at 09:14:45PM +0200, Blue Swirl wrote:
>>  > The previous similar attempt by Anthony for generic DMA using vectored
>>  > IO was abandoned because the malloc/free overhead was more than the
>>
>>
>> Even if there were dynamic allocations in the fast path, the overhead
>>  of malloc/free is nothing if compared to running and waiting a host
>>  kernel syscall to return every 4k, not to tell with O_DIRECT enabled
>>  which is the whole point of having a direct-dma API that truly doesn't
>>  pollute the cache. With O_DIRECT, without a real readv/writev I/O
>>  performance would be destroyed going down to something like 10M/sec
>>  even on the fastest storage/CPU/ram combinations.
>>
>>  So the question is how those benchmarks were run, with or without a
>>  real readv/writev and with or without O_DIRECT to truly eliminate all
>>  CPU cache pollution out of the memory copies?
>>     
>
> I don't know, here's a pointer:
> http://lists.gnu.org/archive/html/qemu-devel/2008-08/msg00092.html
>   

Yes.  A vector IO API is not terribly useful in QEMU without an AIO 
implementation that can make use of it.  posix-aio cannot.

Regards,

Anthony Liguori