On 09/04/2009 01:35 PM, Trond Myklebust wrote:
On Fri, 2009-09-04 at 12:48 -0700, Ben Greear wrote:
I'm trying to optimize a tool that should do NFS reads as fast as possible
from a server in order to stress test the server.
Currently, I open the file as normal and read into a pre-allocated buffer.
This causes a copy of the data to user-space.
Is there any way to cause the nfs client logic to still request the file-read,
but not actually copy anything to user-space?
Maybe some trick with mmap would do this?
How about using O_DIRECT? That just copies the data directly into user
pages and avoids all the overhead of using the page cache?
Note that you can combine O_DIRECT with aio in order to further increase
the speeds.
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.
I haven't looked into aio yet..will go do some googling...
Thanks,
Ben
Cheers
Trond
--
Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc http://www.candelatech.com
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