Packet loss every 30.999 seconds

[David G Lawrence]

> While trying to diagnose a packet loss problem in a RELENG_6 snapshot  
> dated
> November 8, 2007 it looks like I've stumbled across a broken driver or
> kernel routine which stops interrupt processing long enough to severly
> degrade network performance every 30.99 seconds.

   I noticed this as well some time ago. The problem has to do with the
processing (syncing) of vnodes. When the total number of allocated vnodes
in the system grows to tens of thousands, the ~31 second periodic sync
process takes a long time to run. Try this patch and let people know if
it helps your problem. It will periodically wait for one tick (1ms) every
500 vnodes of processing, which will allow other things to run.

Index: ufs/ffs/ffs_vfsops.c
===================================================================
RCS file: /home/ncvs/src/sys/ufs/ffs/ffs_vfsops.c,v
retrieving revision 1.290.2.16
diff -c -r1.290.2.16 ffs_vfsops.c
*** ufs/ffs/ffs_vfsops.c	9 Oct 2006 19:47:17 -0000	1.290.2.16
--- ufs/ffs/ffs_vfsops.c	25 Apr 2007 01:58:15 -0000
***************
*** 1109,1114 ****
--- 1109,1115 ----
  	int softdep_deps;
  	int softdep_accdeps;
  	struct bufobj *bo;
+ 	int flushed_count = 0;
  
  	fs = ump->um_fs;
  	if (fs->fs_fmod != 0 && fs->fs_ronly != 0) {		/* XXX */
***************
*** 1174,1179 ****
--- 1175,1184 ----
  			allerror = error;
  		vput(vp);
  		MNT_ILOCK(mp);
+ 		if (flushed_count++ > 500) {
+ 			flushed_count = 0;
+ 			msleep(&flushed_count, MNT_MTX(mp), PZERO, "syncw", 1);
+ 		}
  	}
  	MNT_IUNLOCK(mp);
  	/*

-DG

David G. Lawrence
President
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