From owner-freebsd-hackers  Fri Oct 30 23:47:15 1998
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To: Mike Smith <mike@smith.net.au>
cc: Peter Jeremy <peter.jeremy@auss2.alcatel.com.au>, hackers@FreeBSD.ORG
Subject: Re: sync writes with softupdates enabled 
In-reply-to: Your message of "Thu, 29 Oct 1998 21:29:10 PST."
             <199810300529.VAA00927@dingo.cdrom.com> 
Date: Sat, 31 Oct 1998 15:46:39 +0800
From: Peter Wemm <peter@netplex.com.au>
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Mike Smith wrote:
> > I've just installed 3.0-RELEASE on the machine destined to be our new
> > news server.  Whilst testing it with soft updates enabled, I'm seeing
> > a lot of sync writes onto /usr:
> > 
> > /dev/wd0s1f on /usr (local, soft-updates, writes: sync 32910 async 4955)
> > 
> > With /usr mounted asynchronously rather than soft updates enabled,
> > I also saw lots of synchronous writes.
> > 
> > The writes are definitely a result of innd - nothing else much is
> > running and pausing innd makes the I/O stop.  I've looked at a
> > ktrace of innd and can't see anything that would obviously cause it
> > to do sync writes (no fsync() or open(O_SYNC) calls).
> > 
> > It's inn-1.7.2 as per the ports with the exception that MMAP is
> > enabled for dbz.
> > 
> > All I can think of is that the MMAP'd history file is causing the
> > kernel to unnecessarily perform sync writes to try and keep the
> > MMAP and VFS images in sync.
> 
> That's almost certainly correct.  Try running without mmap.  It used to 
> be faster that way; maybe it is again?


Hmm, from vm_object.c:

void
vm_object_page_clean(object, start, end, flags)

   pagerflags = (flags & (OBJPC_SYNC | OBJPC_INVAL)) ? VM_PAGER_PUT_SYNC :
0;
   pagerflags |= (flags & OBJPC_INVAL) ? VM_PAGER_PUT_INVAL : 0;
   VOP_FSYNC(vp, NULL, (pagerflags & VM_PAGER_PUT_SYNC)?MNT_WAIT:0, curproc);

vm_object.h:
#define OBJPC_SYNC      0x1                     /* sync I/O */
#define OBJPC_INVAL     0x2                     /* invalidate */

Yet, we have:

vfs_subr.c:             vm_object_page_clean(vp->v_object, 0, 0, TRUE);
vfs_syscalls.c:         vm_object_page_clean(vp->v_object, 0, 0, FALSE);

param.h:#define TRUE    1
param.h:#define FALSE   0

The sync_fsync() writes are calling vfs_msync() with MNT_NOWAIT, but 
vfs_msync() is turning dirty mmap pages into sync writes.  This behavior 
is needed prior to unmount (obviously) but doesn't seem useful for the 
periodic sync.

Cheers,
-Peter




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