K 10
svn:author
V 3
mav
K 8
svn:date
V 27
2015-04-21T10:55:53.025437Z
K 7
svn:log
V 675
Rewrite physio() to not allocate pbufs for unmapped I/O.

pbufs is a limited resource, and their allocator is not SMP-scalable.
So instead of always allocating pbuf to immediately convert it to bio,
allocate bio just here.  If buffer needs kernel mapping, then pbuf is
still allocated, but used only as a source of KVA and storage for a list
of held pages.

On 40-core system doing many 512-byte reads from user level to array of
raw SSDs this change removes huge lock congestion inside pbuf allocator.
It improves peak performance from ~300K to ~1.2M IOPS.  On my previous
24-core system this problem also existed, but was less serious.

Reviewed by:	kib
MFC after:	2 weeks

END