raw readv/writev

[Janet Morgan]

I have two versions of a patch for improving the performance of readv/writev  
for raw io, and I'm not sure which is the better approach.

The current implementation calls read/write for each iovec (unless an override 
routine is defined).  So an 8x32K readv, for example, runs about twice as
slow as a single 256K read.  Both versions of the patch nearly eliminate 
this performance gap.

The first version coalesces the iovecs (up to KIO_MAX_SECTORS bytes of data) 
into a single kiobuf (pre-allocated at file open) and issues 1 call to 
brw_kiovec to submit the io.  The 2nd version of the patch also groups the 
iovecs into a single call to brw_kiovec, but uses one kiobuf per iovec.

Mapping discontiguous virtual memory into a single kiobuf is unconventional,
but minimizes the number of pre-allocated buffer heads (1024 per kiobuf).
It also avoids some of the logistics involved in using one kiobuf for each 
iovec (e.g., when should the kiobufs be allocated/freed and should there
be a system-wide limit on the number of kiobufs in use for this purpose).

Thanks, 
-Janet