[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC 1/2] pci-dma-api-v1
Date: Sun, 30 Nov 2008 16:34:31 -0600
User-agent: Thunderbird (X11/20080925)

Blue Swirl wrote:
On 11/28/08, Andrea Arcangeli <address@hidden> 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:

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


Anthony Liguori

reply via email to

[Prev in Thread] Current Thread [Next in Thread]