syncing large mmaped files

[Tristan Verniquet]

> From: jhb at freebsd.org
> To: freebsd-hackers at freebsd.org
> Subject: Re: syncing large mmaped files
> Date: Thu, 18 Oct 2012 09:39:34 -0400
> CC: kostikbel at gmail.com; tris_vern at hotmail.com
> 
> On Thursday, October 18, 2012 4:35:37 am Konstantin Belousov wrote:
> > On Thu, Oct 18, 2012 at 10:08:22AM +1000, Tristan Verniquet wrote:
> > > 
> > > I want to work with large (1-10G) files in memory but eventually sync
> > > them back out to disk. The problem is that the sync process appears to
> > > lock the file in kernel for the duration of the sync, which can run
> > > into minutes. This prevents other processes from reading from the file
> > > (unless they already have it mapped) for this whole time. Is there
> > > any way to prevent this? I think I read in a post somewhere about
> > > openbsd implementing partial-writes when it hits a file with lots of
> > > dirty pages in order to prevent this. Is there anything available for
> > > FreeBSD or is there another way around it?
> > >
> > No, currently the vnode lock is held exclusive for the whole duration
> > of the msync(2) syscall or its analog from the syncer.
> > 
> > Making a change to periodically drop the vnode lock in
> > vm_object_page_clean() might be possible, but requires the benchmarking
> > to make sure that we do not pessimize the common case. Also, this opens
> > a possibility for the vnode reclamation meantime.
> 
> You can simulate this in userland by breaking up your msync() into multiple
> msync() calls where each call just syncs a portion of the file.

Thanks, I was doing this and I thought I was getting much worse performance from the msync over the fsync, however I am trying it again now and the difference doesn't seem as large as I first imagined. It is still taking about 4x as long for the case where all the pages are dirty but catches up when the file is more sparsely written. I guess that is probably acceptable.

When all pages are dirty, iostat shows that the fsync will write 128KB/Transaction, whereas msync always does 16 KB/Transaction and a lower MB/s. It will continue to do this if I only dirty every 2nd, 3rd or 4th page. When I only dirty every 5th page the fsync seems to kick into another mode and starts doing 16KB/Transaction and the time starts becoming comparable to msync.

Is there anyway to get that fsync 128K/Transaction performance increase when all pages are dirty with msync? 


> > Anyway, note that you cannot 'work with large files in memory', even if
> > you have enough RAM and no pressure to hold all the file pages resident.
> > The syncer will do a writeback periodically regardless of the application
> > calling msync(2) or not, with the interval of approximately 30 seconds.
> 
> You can mmap with MAP_NOSYNC to prevent the syncer from writing the file out
> every 30 seconds.

Yes, I was mapping MAP_NOSYNC.
 
> -- 
> John Baldwin
> _______________________________________________
> freebsd-hackers at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
> To unsubscribe, send any mail to "freebsd-hackers-unsubscribe at freebsd.org"