[FFM performance] Intermediate buffer allocation when returning structs

Thu Jan 16 17:25:55 UTC 2025

Hello Matthias,

We've been exploring this direction internally as well. As you've found, 
downcall handles/upcall stubs sometimes need to allocate memory. The 
return buffer case that you've run into is one such case, others are: 
when a struct that does not fit into a single register is passed by 
value on Windows, we need to create a copy. When a struct is passed by 
value to an upcall stub, we need to allocate memory to hold the value.

I took a look at your patch. One of the problems I see with a 
one-element cache is that some upcall stubs might never benefit from it, 
since a preceding downcall already claimed the cache. Though, I believe 
a chain of downcalls and upcalls is comparatively rare. A two element 
cache might be better. That way a sequence of downcall -> upcall, that 
both use by-value structs, will be able to benefit from the cache.

Having a cache per carrier thread is probably a good idea. A cache per 
thread is also possibly an option, if the overhead seems acceptable (the 
cache is only initialized for threads that actually call native code 
after all). This would also be a little faster, I think.

One thing that's unfortunate is the use of a shared arena, even in the 
fallback case, since closing that is very slow. Another problem is that 
with your current implementation, we are no longer tracking the lifetime 
of the memory correctly, and it is possible to access memory that was 
already returned to the cache. Using a proper lifetime (i.e. 
creating/closing a new arena per call) has helped to catch bugs in the 
past. If we want to keep doing that, we'd have to re-wrap the memory of 
the cache with a new arena (using MemorySegment::reinterpret), which we 
then close after a downcall, to return elements to the cache. I suggest 
restructuring the code so that it always creates  a new confined arena, 
as today, but then either: 1) grabs a memory segment from the cache, and 
attaches that to the new confined arena (using MS::reintrepret), or 2) 
in the case of a cache miss, just allocates a new segment from the 
confined arena we created.

WDYT?

Jorn

On 16-1-2025 11:00, Matthias Ernst wrote:
> Hi, I noticed a source of overhead when calling foreign functions with 
> small aggregate return values.
>
> For example, a function returning a struct Vector2D { double x; double 
> y } will cause a malloc/free inside the downcall handle on every call. 
> On my machine, this accounts for about 80% of the call overhead.
>
> Choice stack:
> |java.lang.Thread.State: RUNNABLE *at 
> jdk.internal.misc.Unsafe.allocateMemory0(java.base at 25-ea/Native 
> Method) *... *at 
> jdk.internal.foreign.abi.SharedUtils.newBoundedArena(java.base at 25-ea/SharedUtils.java:386) 
> *	at 
> jdk.internal.foreign.abi.DowncallStub/0x000001f001084c00.invoke(java.base at 25-ea/Unknown 
> Source) at 
> java.lang.invoke.DirectMethodHandle$Holder.invokeStatic(java.base at 25-ea/DirectMethodHandle$Holder) 
> |
> While it might be difficult to eliminate these intermediate buffers, I 
> would propose to try reusing them.
>
> What's happening here:
> * the ARM64 ABI returns such a struct in two 128 bit registers v0/v1 [0]
> * the VM stub calling convention around this expects an output buffer 
> to copy v0/v1 into:  [1]
> stub(out) { ... out[0..16) = v0; out[16..32) = v1; }
> * the FFM downcall calling convention OTOH expects a user-provided 
> SegmentAllocator to allocate a 16 byte StructLayout(JAVA_DOUBLE, 
> JAVA_DOUBLE). The generated method handle to adapt to the stub looks 
> roughly like this [2]:
>  ffm(allocator) {
> *  tmp = malloc(32)*
>   stub(tmp)
>   result = allocator.allocate(16)
>   result[0..8) = tmp[0..8)
>   result[8..16) = tmp[16..24)
> *  free(tmp)*
>   return result
> }
>
> Now there's an easy way around this for the user by using a different 
> native signature:
> void g(Vector2D *out) { *out = f(); }
> This eliminates the intermediate buffer altogether.
>
> However, if we wanted to optimize the return-by-value path, I can 
> think of three options:
> * enhance the stub calling conventions to directly copy only the 
> narrowed output registers into the result buffer.  This looks rather 
> involved.
> * allocate the tmp buffer using the user's allocator as well (e.g. in 
> conjunction with the result + slicing). The Linker api is somewhat 
> lenient about how `allocator` will be exactly invoked: "used by the 
> linker runtime to allocate the memory region associated with the 
> struct returned by the downcall method handle".  However, this may be 
> surprising to the caller.
> * keep the tmp buffer allocation internal, but optimize it. This is 
> what I'm proposing here.
>
> A possible counter-argument could be "this is just one allocation out 
> of two". However, the user has control over `allocator`, and may 
> re-use the same segment across calls, but they have no control over 
> the tmp allocation.
>
> I've worked on a patch that takes this last route, using a one-element 
> thread-local cache: https://github.com/openjdk/jdk/pull/23142, it 
> reduces call time from 36->8ns / op on my machine and I observe no 
> more GC's.
>
> Would there be interest in pursuing this?
>
> Thx
> Matthias
>
>
> [0] 
> https://learn.microsoft.com/en-us/cpp/build/arm64-windows-abi-conventions?view=msvc-170#return-values
> [1] 
> https://github.com/openjdk/jdk/blob/9c430c92257739730155df05f340fe144fd24098/src/java.base/share/classes/jdk/internal/foreign/abi/CallingSequenceBuilder.java#L97
> [2] "binding context": 
> https://github.com/openjdk/jdk/blob/9c430c92257739730155df05f340fe144fd24098/src/java.base/share/classes/jdk/internal/foreign/abi/BindingSpecializer.java#L296
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