Re: Reading NFS file without copying to user-space?

[Ben Greear <greearb@candelatech.com>]

On 09/04/2009 01:58 PM, Trond Myklebust wrote:
> On Sep 4, 2009, at 16:49, Ben Greear <greearb@candelatech.com> wrote:
>
>> I'm using O_DIRECT (so that the server is continually stressed even if
>> the file would have otherwise been cached locally on the client).
>>
>> This still causes a copy of the contents to user-space when I do a
>> read() call though, as far as I can tell. Since I'm normally not looking
>> at this data at all, the memory copy from kernel to user is wasted
>> effort in my case.
>
> You're missing the point. O_DIRECT does not copy data from the kernel
> into userspace. The data is placed directly into the user buffer from
> the socket.

I may be going about things all wrong...

>
> The only faster alternative would be to directly discard the data in the
> socket, and we offer no option to do that.

I'm opening an fd like this:


          uint32 flgs = O_RDONLY | O_DIRECT | O_LARGEFILE;
          fd = open(fname, flgs);

Then read from the fd it:
          int retval = read(fd, rcv_buffer_ptr, my_read_len);

rcv_buffer_ptr is just a 1MB (or so) array of bytes.


Maybe I need to use aio_read with O_DIRECT to get the benefits you speak of?

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com