Re: [PATCH] remove throttle_vm_writeout()

[Miklos Szeredi <miklos@szeredi.hu>]

> > > This is a somewhat general problem: a userspace process is in the IO path. 
> > > Userspace block drivers, for example - pretty much anything which involves
> > > kernel->userspace upcalls for storage applications.
> > > 
> > > I solved it once in the past by marking the userspace process as
> > > PF_MEMALLOC and I beleive that others have implemented the same hack.
> > > 
> > > I suspect that what we need is a general solution, and that the solution
> > > will involve explicitly telling the kernel that this process is one which
> > > actually cleans memory and needs special treatment.
> > > 
> > > Because I bet there will be other corner-cases where such a process needs
> > > kernel help, and there might be optimisation opportunities as well.
> > > 
> > > Problem is, any such mark-me-as-special syscall would need to be
> > > privileged, and FUSE servers presently don't require special perms (do
> > > they?)
> > 
> > No, and that's a rather important feature, that I'd rather not give
> > up.
> 
> Can fuse do it?  Perhaps the fs can diddle the server's task_struct at
> registration time?

No, it's futile.  What if another process is involved (ssh in case of
sshfs), etc.

> >  But with the dirty limiting, the memory cleaning really shouldn't
> > be a problem, as there is plenty of memory _not_ used for dirty file
> > data, that the filesystem can use during the writeback.
> 
> I don't think I understand that.  Sure, it _shouldn't_ be a problem.  But it
> _is_.  That's what we're trying to fix, isn't it?

The problem, I believe is in the memory allocation code, not in fuse.

In the example, memory allocation may be blocking indefinitely,
because we have 4MB under writeback, even though 28MB can still be
made available.  And that _should_ be fixable.

> > So the only thing the kernel should be careful about, is not to block
> > on an allocation if not strictly necessary.
> > 
> > Actually a trivial fix for this problem could be to just tweak the
> > thresholds, so to make the above scenario impossible.  Although I'm
> > still not convinced, this patch is perfect, because the dirty
> > threshold can actually change in time...
> > 
> > Index: linux/mm/page-writeback.c
> > ===================================================================
> > --- linux.orig/mm/page-writeback.c      2007-10-05 00:31:01.000000000 +0200
> > +++ linux/mm/page-writeback.c   2007-10-05 00:50:11.000000000 +0200
> > @@ -515,6 +515,12 @@ void throttle_vm_writeout(gfp_t gfp_mask
> >          for ( ; ; ) {
> >                 get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
> > 
> > +               /*
> > +                * Make sure the theshold is over the hard limit of
> > +                * dirty_thresh + ratelimit_pages * nr_cpus
> > +                */
> > +               dirty_thresh += ratelimit_pages * num_online_cpus();
> > +
> >                  /*
> >                   * Boost the allowable dirty threshold a bit for page
> >                   * allocators so they don't get DoS'ed by heavy writers
> 
> I can probably kind of guess what you're trying to do here.  But if
> ratelimit_pages * num_online_cpus() exceeds the size of the offending zone
> then things might go bad.

I think the admin can do quite a bit of other damage, by setting
dirty_ratio too high.

Maybe this writeback throttling should just have a fixed limit of 80%
ZONE_NORMAL, and limit dirty_ratio to something like 50%.

Miklos
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