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Re: [PATCH] util/async: make bh_aio_poll() O(1)
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH] util/async: make bh_aio_poll() O(1) |
Date: |
Fri, 21 Feb 2020 13:47:42 +0000 |
On Wed, Feb 19, 2020 at 08:05:12PM +0100, Paolo Bonzini wrote:
> Il mer 19 feb 2020, 18:58 Stefan Hajnoczi <address@hidden> ha scritto:
>
> > On Wed, Feb 19, 2020 at 12:09:48PM +0100, Paolo Bonzini wrote:
> > > Really a great idea, though I have some remarks on the implementation
> > below.
> > >
> > > On 19/02/20 11:00, Stefan Hajnoczi wrote:
> > > > + * Each aio_bh_poll() call carves off a slice of the BH list. This
> > way newly
> > > > + * scheduled BHs are not processed until the next aio_bh_poll()
> > call. This
> > > > + * concept extends to nested aio_bh_poll() calls because slices are
> > chained
> > > > + * together.
> > >
> > > This is the tricky part so I would expand a bit on why it's needed:
> > >
> > > /*
> > > * Each aio_bh_poll() call carves off a slice of the BH list, so that
> > > * newly scheduled BHs are not processed until the next aio_bh_poll()
> > > * call. All active aio_bh_poll() calls chained their slices together
> > > * in a list, so that nested aio_bh_poll() calls process all scheduled
> > > * bottom halves.
> > > */
> >
> > Thanks, will fix in v2.
> >
> > > > +struct BHListSlice {
> > > > + QEMUBH *first_bh;
> > > > + BHListSlice *next;
> > > > +};
> > > > +
> > >
> > > Using QLIST and QSLIST removes the need to create your own lists, since
> > > you can use QSLIST_MOVE_ATOMIC and QSLIST_INSERT_HEAD_ATOMIC. For
> > example:
> > >
> > > struct BHListSlice {
> > > QSLIST_HEAD(, QEMUBH) first_bh;
> > > QLIST_ENTRY(BHListSlice) next;
> > > };
> > >
> > > ...
> > >
> > > QSLIST_HEAD(, QEMUBH) active_bh;
> > > QLIST_HEAD(, BHListSlice) bh_list;
> >
> > I thought about this but chose the explicit tail pointer approach
> > because it lets aio_compute_timeout() and aio_ctx_check() iterate over
> > both the active BH list and slices in a single for loop :). But
> > thinking about it more, maybe it can still be done by replacing
> > active_bh with a permanently present first BHListSlice element.
> >
>
> Probably not so easy since the idea was to empty the slices list entirely
> via the FIFO order.
>
> But since you are rewriting everything anyway, can you add a flag for
> whether there are any non-idle bottom halves anywhere in the list? It need
> not be computed perfectly, because any non-idle bh will cause the idle
> bottom halves to be triggered as well; you can just set in qemu_bh_schedule
> and clear it at the end of aio_bh_poll.
>
> Then if there is any active bh or slice you consult the flag and use it to
> return the timeout, which will be either 0 or 10ms depending on the flag.
> That's truly O(1). (More precisely, this patch goes from O(#created-bh) to
> O(#scheduled-bh), which of course is optimal for aio_bh_poll but not for
> aio_compute_timeout; making timeout computation O(1) can lower latency a
> bit by decreasing the constant factor).
Yes, good idea. I'll send a follow-up patch.
Stefan
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