[llvm-dev] RFC: Sanitizer-based Heap Profiler

Tue Jul 13 11:12:46 PDT 2021

On Mon, Jul 12, 2021 at 6:40 PM Wenlei He <wenlei at fb.com> wrote:

> This is a big undertaking with good potential but also some uncertainty on
> how effective such optimizations are for larger workloads, so really
> appreciate the pioneering effort in LLVM.
>
>
>
> We (facebook) are very interested in this too. I've reached out to David
> and Teresa a while ago about this, and was going to wait for the RFC before
> having more detailed discussions. But now that we're discussing it, here’s
> my two cents about the responsibility division between compiler and
> allocator, and the API.
>
>
>
> I think it'd be beneficial if we let compiler do more heavy lighting
> instead of relying heavily on allocator. If we rely on less magic inside an
> allocator, we will likely benefit more users who may use different
> allocators. Otherwise there's a risk that the compiler part may be too
> coupled with a specific allocator, which limits the overall effectiveness
> of PGHO outside of that allocator.
>

I agree with this in general.  At a  high level, there are multiple
different types of locality optimizations each requiring different
treatment for the peak performance:

1) Locality policies fully enforced by the compiler. Examples include
co-allocation of different types of small sized objects with the same
lifetime (which is statically determinable).
2) Locality policies determined by the compiler but provided as hints to
the allocator(s).   Examples include coarse grain locality optimizations as
well as co-allocations for objects whose lifetime can not be proved to be
identical statically.  The allocator provides APIs to pass the hints, but
can choose to ignore them.  Due to the freelists and fragmentation, there
is no guarantee two small objects in the same locality group (compiler
hint) will be allocated in the same cacheline.
3) A hybrid of those -- there are some compiler transformations needed but
also relying on collaboration from the allocator. An example is struct
splitting.  Blindly splitting the struct without hint to the runtime won't
prevent the cold parts from polluting the cache.

>
>
> This also affects what we want to expose in the new API for hinting the
> allocator (e.g. provide grouping or arena-like hint computed by compiler
> vs. passing a number of factors through the API which would help compute
> that inside allocator). With a general, stable API, hope we won't need to
> change API when we want to take more runtime info (temporal etc., even just
> for experiments) into account, or when we improve and leverage more from
> compiler analysis (I agree that in the long run we should improve compiler
> analysis).
>

Agree.

>
>
> I've talked with jemalloc folks on our side, and we're flexible to API
> changes. In this case, it makes sense to avoid abstraction overhead from
> wrappers.
>
>
>

yes. A mature allocator is tuned for years to reduce the allocation
overhead as well as minimizing the memory fragmentation. Duplicating those
in the wrappers can be a huge task -- except for special cases.

thanks,

David

> Looking forward to the RFC and more discussions on this.
>
>
>
> Thanks,
>
> Wenlei
>
>
>
> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Xinliang
> David Li via llvm-dev <llvm-dev at lists.llvm.org>
> *Date: *Thursday, July 8, 2021 at 9:55 AM
> *To: *Andrey Bokhanko <andreybokhanko at gmail.com>
> *Cc: *llvm-dev <llvm-dev at lists.llvm.org>
> *Subject: *Re: [llvm-dev] RFC: Sanitizer-based Heap Profiler
>
>
>
>
>
> On Thu, Jul 8, 2021 at 8:03 AM Andrey Bokhanko <andreybokhanko at gmail.com>
> wrote:
>
> Hi Teresa,
>
>
>
> One more thing, if you don't mind.
>
>
>
> On Tue, Jul 6, 2021 at 12:54 AM Teresa Johnson <tejohnson at google.com>
> wrote:
>
> We initially plan to use the profile information to provide guidance to
> the dynamic allocation runtime on data allocation and placement. We'll send
> more details on that when it is fleshed out too.
>
>
>
> I played with the current implementation, and became a bit concerned if
> the current data profile is sufficient for an efficient data allocation
> optimization.
>
>
>
>
>
> First, there is no information on temporal locality -- only total_lifetime
> of an allocation block is recorded, not start / end times -- let alone
> timestamps of actual memory accesses. I wonder what criteria would be used
> by data profile-based allocation runtime to allocate two blocks from the
> same memory chunk?
>
>
>
> First, I think per-allocation start-end time should be added to
> approximate temporal locality.
>
>
>
> Detailed temporal locality information is not tracked is by design for a
> various of reasons:
>
>
>
> 1.  This can be done with static analysis. The idea is for the compiler to
> instrument a potentially hot access region and profile the start and end
> address of the accessed memory regions. This information can be combined
> with the regular heap profile data. In profile-use phase, the compiler can
> perform access pattern analysis and produce affinity graph
>
>
>
> 2.  We try to make use of existing allocator runtime (tcmalloc) for
> locality optimization. The runtime has been tuned for years to have the
> most efficient code for fast-path allocation.  For hot allocation sites,
> adding too much overhead (e.g. via wrapper etc) can lead to overhead that
> totally eat up the gains from the locality optimization;
>
>
>
> 3. tcmalloc currently uses size class based partitioning, which makes
> co-allocation of small objects of different size classes impossible. Even
> for objects with the same type/size, due to the use of free lists, there is
> no guarantee that consecutively allocated objects are placed together.
>
>
>
> 4. a bump-pointer allocator has its own sets of problems -- when not used
> carefully, it can lead to huge memory waste due to fragmentation.  In
> reality it only helps grouping for initial set of allocations when pointer
> bumps continuously -- during stable state, the allocations will also be all
> over the place and no contiguity can be guaranteed.
>
>
>
> This is why initially we focus more coarse grain locality optimization --
> 1) co-placement to improve DTLB performance and 2) improving dcache
> utilization using only lifetime and hotness information.
>
>
>
> Longer term, we need to beef up compiler based analysis -- objects with
> the exact life times can be safely co-allocated via compiler based
> transformation. Also objects with similar lifetimes can be co-allocated
> without introducing too much fragmentation.
>
>
>
>
>
> Thanks,
>
>
>
> David
>
>
>
>
>
> Second, according to the data from [Savage'20], memory accesses affinity
> (= space distance between temporarily close memory accesses from two
> different allocated blocks) is crucial: figure #12 demonstrates that this
> is vital for omnetpp benchmark from SPEC CPU 2017.
>
>
>
> Said this, my concerns are based essentially on a single paper that
> employs specific algorithms to guide memory allocation and measures their
> impact on a specific set of benchmarks. I wonder if you have preliminary
> data that validates sufficiency of the implemented data profile for
> efficient optimization of heap memory allocations?
>
>
>
> References:
>
> [Savage'20] Savage, J., & Jones, T. M. (2020). HALO: Post-Link Heap-Layout
> Optimisation. CGO 2020: Proceedings of the 18th ACM/IEEE International
> Symposium on Code Generation and Optimization,
> https://doi.org/10.1145/3368826.3377914
>
>
>
> Yours,
>
> Andrey
>
>
>
>
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