Subject: Re: [PATCH 03/12] reftable/merged: advance subiter on subsequent iteration

From: Patrick Steinhardt <[email-masked]>
Date: Tue, 27 Feb 2024 17:50:01 +0100

On Tue, Feb 20, 2024 at 12:25:10PM -0600, Justin Tobler wrote:
> On 24/02/14 08:45AM, Patrick Steinhardt wrote:
> > When advancing the merged iterator, we pop the top-most entry from its
> 
> s/top-most/topmost
> 
> > priority queue and then advance the sub-iterator that the entry belongs
> > to, adding the result as a new entry. This is quite sensible in the case
> > where the merged iterator is used to actual iterate through records. But
> 
> s/actual/actually
> 
> > the merged iterator is also used when we look up a single record, only,
> > so advancing the sub-iterator is wasted effort because we would never
> > even look at the result.
> > 
> > Instead of immediately advancing the sub-iterator, we can also defer
> > this to the next iteration of the merged iterator by storing the
> > intent-to-advance. This results in a small speedup when reading many
> > records. The following benchmark creates 10000 refs, which will also end
> > up with many ref lookups:
> > 
> >     Benchmark 1: update-ref: create many refs (revision = HEAD~)
> >       Time (mean ± σ):     337.2 ms ±   7.3 ms    [User: 200.1 ms, System: 136.9 ms]
> >       Range (min … max):   329.3 ms … 373.2 ms    100 runs
> > 
> >     Benchmark 2: update-ref: create many refs (revision = HEAD)
> >       Time (mean ± σ):     332.5 ms ±   5.9 ms    [User: 197.2 ms, System: 135.1 ms]
> >       Range (min … max):   327.6 ms … 359.8 ms    100 runs
> > 
> >     Summary
> >       update-ref: create many refs (revision = HEAD) ran
> >         1.01 ± 0.03 times faster than update-ref: create many refs (revision = HEAD~)
> > 
> > While this speedup alone isn't really worth it, this refactoring will
> > also allow two additional optimizations in subsequent patches. First, it
> > will allow us to special-case when there is only a single sub-iter left
> > to circumvent the priority queue altogether. And second, it makes it
> > easier to avoid copying records to the caller.
> > 
> > Signed-off-by: Patrick Steinhardt <[email-masked]>
> > ---
> >  reftable/merged.c | 26 ++++++++++++--------------
> >  1 file changed, 12 insertions(+), 14 deletions(-)
> > 
> > diff --git a/reftable/merged.c b/reftable/merged.c
> > index 12ebd732e8..9b1ccfff00 100644
> > --- a/reftable/merged.c
> > +++ b/reftable/merged.c
> > @@ -19,11 +19,12 @@ license that can be found in the LICENSE file or at
> >  
> >  struct merged_iter {
> >  	struct reftable_iterator *stack;
> > +	struct merged_iter_pqueue pq;
> >  	uint32_t hash_id;
> >  	size_t stack_len;
> >  	uint8_t typ;
> >  	int suppress_deletions;
> > -	struct merged_iter_pqueue pq;
> > +	ssize_t advance_index;
> >  };
> >  
> >  static int merged_iter_init(struct merged_iter *mi)
> > @@ -96,13 +97,17 @@ static int merged_iter_next_entry(struct merged_iter *mi,
> >  	struct pq_entry entry = { 0 };
> >  	int err = 0;
> >  
> > +	if (mi->advance_index >= 0) {
> > +		err = merged_iter_advance_subiter(mi, mi->advance_index);
> > +		if (err < 0)
> > +			return err;
> > +		mi->advance_index = -1;
> > +	}
> > +
> 
> Without additional context, it isn't immediately clear to me why the
> sub-iterator is condionally advanced at the beginning. Maybe a comment
> could be added to explain as done in the commit message to help with
> clarity?

I tried to mention this in the commit message with the last paragraph.
Adding a comment doesn't make much sense at this point in the patch
seires because a later patch changes how this works.

> >  	if (merged_iter_pqueue_is_empty(mi->pq))
> >  		return 1;
> >  
> >  	entry = merged_iter_pqueue_remove(&mi->pq);
> > -	err = merged_iter_advance_subiter(mi, entry.index);
> > -	if (err < 0)
> > -		return err;
> >  
> >  	/*
> >  	  One can also use reftable as datacenter-local storage, where the ref
> > @@ -116,14 +121,6 @@ static int merged_iter_next_entry(struct merged_iter *mi,
> >  		struct pq_entry top = merged_iter_pqueue_top(mi->pq);
> >  		int cmp;
> >  
> > -		/*
> > -		 * When the next entry comes from the same queue as the current
> > -		 * entry then it must by definition be larger. This avoids a
> > -		 * comparison in the most common case.
> > -		 */
> > -		if (top.index == entry.index)
> > -			break;
> > -
> 
> I'm not quite sure I follow by the above check is removed as part of
> this change. Would you mind clarifying?

The loop that this comparison has been part of was popping all entries
from the priority queue that are being shadowed by the sub-iterator from
which we're about to return the entry. So e.g. in the case of a ref
record, we discard all records with the same refname which are shadowed
by a newer (higher update-index) table.

The removed condition was an optimization was a micro-optimization: when
the next entry in the pqueue is from the same index as the entry we are
about to return, then we know that it cannot have been shadowed. This
allowed us to avoid a key comparison.

But with the change in this commit we don't even add the next record of
the current sub-iter to the pqueue, and thus the condition cannot happen
anymore.

Patrick