Re: [PATCH 10/10] mm: per device dirty threshold

[Miklos Szeredi <miklos@szeredi.hu>]

> > This is probably a
> >  reasonable thing to do but it doesn't feel like the right place.  I
> >  think get_dirty_limits should return the raw threshold, and
> >  balance_dirty_pages should do both tests - the bdi-local test and the
> >  system-wide test.
> 
> Ok, that makes sense I guess.

Well, my narrow minded world view says it's not such a good idea,
because it would again introduce the deadlock scenario, we're trying
to avoid.

In a sense allowing a queue to go over the global limit just a little
bit is a good thing.  Actually the very original code does that: if
writeback was started for "write_chunk" number of pages, then we allow
"ratelimit" (8) _new_ pages to be dirtied, effectively ignoring the
global limit.

That's why I've been saying, that the current code is so unfair: if
there are lots of dirty pages to be written back to a particular
device, then balance_dirty_pages() allows the dirty producer to make
even more pages dirty, but if there are _no_ dirty pages for a device,
and we are over the limit, then that dirty producer is allowed
absolutely no new dirty pages until the global counts subside.

I'm still not quite sure what purpose the above "soft" limiting
serves.  It seems to just give advantage to writers, which managed to
accumulate lots of dirty pages, and then can convert that into even
more dirtyings.

Would it make sense to remove this behavior, and ensure that
balance_dirty_pages() doesn't return until the per-queue limits have
been complied with?

Miklos

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