[Rd] Recycling memory with a small free list

[luke-tierney at uiowa.edu]

On Wed, 18 Feb 2015, Nathan Kurz wrote:

> On Wed, Feb 18, 2015 at 7:19 AM, Radford Neal <radford at cs.toronto.edu> wrote:
>>> ... with assignments inside of loops like this:
>>>
>>> reweight = function(iter, w, Q) {
>>>   for (i in 1:iter) {
>>>     wT = w * Q
>>>   }
>>> }
>>> ... before the RHS is executed, the LHS allocation would be added
>>> to a small fixed length list of available space which is checked
>>> before future allocations.   If the same size is requested before the
>>> next garbage collection, the allocation is short-circuited and the
>>> allocation is reused.   This list could be very small, possibly even
>>> only a single entry.  Entries would only be put on the list if they
>>> have no other references.
>
> Here's an article about the benefits of this approach in Go that might
> explain better than I was able:
> https://blog.cloudflare.com/recycling-memory-buffers-in-go/
> Their charts explain the goal very clearly: stabilize at a smaller
> amount of memory to reduce churn, which improves performance in a
> myriad of ways.

Thanks -- will have a look.

>> Reusing the LHS storage immediately isn't possible in general, because
>> evaluation of the RHS might produce an error, in which case the LHS
>> variable is supposed to be unchanged.
>
> What's the guarantee R actually makes?  What's an example of the use
> case where this behaviour would be required? More generally, can one
> not assume "a = NULL; a = func()" is equivalent to "a = func()" unless
> func() references 'a' or has it as an argument?  Or is the difficulty
> that there is no way to know in advance it if will be referenced?
>
>> Detecting special cases where
>> there is guaranteed to be no error, or at least no error after the
>> first modification to newly allocated memory, might be too
>> complicated.
>
> Yes, if required, the complexity of guaranteeing this might  well rule
> out the approach I suggested.
>
>> Putting the LHS storage on a small free list for later reuse (only
>> after the old value of the variable will definitely be replaced) seems
>> more promising (then one would need only two copies for examples such
>> as above, with them being used in alternate iterations).
>
> OK, let's consider that potentially easier option instead:  do nothing
> immediately, but add a small queue for recycling from which the
> temporary might be drawn.   It has slightly worse cache behavior, but
> should handle most of the issues with memory churn.
>
>> However,
>> there's a danger of getting carried away and essentially rewriting
>> malloc.  To avoid this, one might try just calling "free" on the
>> no-longer-needed object, letting "malloc" then figure out when it can
>> be re-used.
>
> Yes, I think that's what I was anticipating:  add a free() equivalent
> that does nothing if the object has multiple references/names, but
> adds the object to small fixed size "free list" if it does not.
> Perhaps this is only for certain types or for objects above a certain
> size.
>
> When requesting memory, allocvector() or perhaps R_alloc() does a
> quick check of that "free list" to see if it has anything of the exact
> requested size.  If it does, it short circuits and recycles it.  If it
> doesn't, normal allocation takes place.
>
> The "free list" is stored as two small fixed size arrays containing
> size/address pairs.   Searching is done linearly using code that
> optimizes to SIMD comparisons.   For 4/8/16 slots overhead of the
> search should be unmeasurably fast.
>
> The key to the approach would be keeping it simple, and realizing that
> the goal is only to get the lowest hanging fruit:  repeated
> assignments of large arrays used in a loop.  If it's complex, skip it
> --- the behavior will be no worse than current.
>
> By the way, what's happening with Luke's refcnt patches?  From the
> outside, they seem like a great improvement.
> http://homepage.stat.uiowa.edu/~luke/talks/dsc2014.pdf
> http://developer.r-project.org/Refcnt.html
> Are they slated to become the standard approach?  Are they going to be dropped?
> Will both approaches be kept in parallel?

The approach can be enabled in R-devel by defining a preprocessor
variable.  It's about 90% of where it needs to be to become the
default. I had to put work on hold for a while but will be getting
back to it soon. It's too late to turn on for 3.2.0 due in April, but
I'm hopeful of switching to reference counting in R-devel by August or
so.

>
>> Unfortunately, that seems not to be safe, because it's
>> possible that there is a reference to the no-longer-needed object on
>> the PROTECT stack, even though no one should actually be looking at
>> it any more.
>
> Can you explain this case?   I don't think I understand it.
>
>> In the current version of pqR (see pqR-project.org), modifications are
>> (often) done in place for statements such as w = w * Q, but not
>> curretly when the LHS variable does not appear on the RHS.
>
> Yes, I looked at it earlier, and was excited to see that Luke had
> ported half of your approach to standard R:
> https://github.com/wch/r-source/blob/trunk/src/main/arithmetic.h#L65
>
> But only the RHS temporary variables optimizations made it over. Your
> LHS "w = w * Q" optimizations did not, but I didn't see any discussion
> of why.   Was
> it attempted and issues were found?
>
> I think what I'm suggesting is complementary to that.   Direct reuse
> is best if it can be detected, but recycling will provide more
> opportunities for optimization.  Of course, what I'm suggesting is
> always quite obvious, and I presume it's part what he includes in the
> slide in his talk that mentions "Explore releasing memory when
> reference count drops to zero".

This is part of the missing 10% of things I 'd like to explore before
going live. Releasing large (malloc'ed) objects with reference counts
that hit zero back to the malloc system is probably not to hard to get
right. Holding onto these objects in a free list might be worth
looking into, but as Radford suggests a good malloc may be good enough
at doing that already.

Best,

luke

>
> --nate
>
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-- 
Luke Tierney
Ralph E. Wareham Professor of Mathematical Sciences
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
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