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From: Gleb Smirnoff <glebius@FreeBSD.org>
Date: Mon, 4 Jun 2012 12:49:22 +0000 (UTC)
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Subject: svn commit: r236560 - head/sys/kern
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Author: glebius
Date: Mon Jun  4 12:49:21 2012
New Revision: 236560
URL: http://svn.freebsd.org/changeset/base/236560

Log:
  Optimise kern_sendfile(): skip cycling through the entire mbuf chain in
  m_cat(), storing pointer to last mbuf in chain in local variable and
  attaching new mbuf to the end of chain.
  
  Submitter reports that CPU load dropped for > 10% on a web server
  serving large files with this optimisation.
  
  Submitted by:	Sergey Budnevitch <sb nginx.com>

Modified:
  head/sys/kern/uipc_syscalls.c

Modified: head/sys/kern/uipc_syscalls.c
==============================================================================
--- head/sys/kern/uipc_syscalls.c	Mon Jun  4 12:36:58 2012	(r236559)
+++ head/sys/kern/uipc_syscalls.c	Mon Jun  4 12:49:21 2012	(r236560)
@@ -1962,6 +1962,7 @@ kern_sendfile(struct thread *td, struct 
 	 * and takes care of the overall progress.
 	 */
 	for (off = uap->offset, rem = uap->nbytes; ; ) {
+		struct mbuf *mtail = NULL;
 		int loopbytes = 0;
 		int space = 0;
 		int done = 0;
@@ -2181,10 +2182,15 @@ retry_space:
 			m0->m_len = xfsize;
 
 			/* Append to mbuf chain. */
-			if (m != NULL)
-				m_cat(m, m0);
-			else
-				m = m0;
+			if (mtail != NULL) {
+				mtail->m_next = m0;
+			} else {
+				if (m != NULL)
+					m_cat(m, m0);
+				else
+					m = m0;
+			}
+			mtail = m0;
 
 			/* Keep track of bits processed. */
 			loopbytes += xfsize;