ZFS txg implementation flaw

[Slawa Olhovchenkov]

On Mon, Oct 28, 2013 at 02:22:16PM -0700, Jordan Hubbard wrote:

> 
> On Oct 28, 2013, at 2:28 AM, Slawa Olhovchenkov <slw at zxy.spb.ru> wrote:
> 
> > As I see ZFS cretate seperate thread for earch txg writing.
> > Also for writing to L2ARC.
> > As result -- up to several thousands threads created and destoyed per
> > second. And hundreds thousands page allocations, zeroing, maping
> > unmaping and freeing per seconds. Very high overhead.
> 
> How are you measuring the number of threads being created / destroyed?   This claim seems erroneous given how the ZFS thread pool mechanism actually works (and yes, there are thread pools already).
> 
> It would be helpful to both see your measurement methodology and the workload you are using in your tests.

Semi-indirect.
dtrace -n 'fbt:kernel:vm_object_terminate:entry { @traces[stack()] = count(); }'

After some (2-3) seconds 

              kernel`vnode_destroy_vobject+0xb9
              zfs.ko`zfs_freebsd_reclaim+0x2e
              kernel`VOP_RECLAIM_APV+0x78
              kernel`vgonel+0x134
              kernel`vnlru_free+0x362
              kernel`vnlru_proc+0x61e
              kernel`fork_exit+0x11f
              kernel`0xffffffff80cdbfde
             2490

I don't have user process created threads nor do fork/exit.