[Gc] Alignment/padding of small objects on 64-bit platform (x86-64)

[Stefan Kral]

Hello.

My app operates on a lot of 3-word objects and I wonder if it would be
reasonable to adapt the granule size accordingly.

The application requires 64-bit pointers and has no double-word alignment
requirements, so I would be keen on reducing the memory requirement.

I'm grateful for any advise and/or tips!

Best Regards,
Stefan.

[Bruce Hoult]

The GC needs to store a pointer for a free list, so needs at least 8 bytes
per object. And aligned on most CPUs.

Post by Stefan Kral
Hello.
My app operates on a lot of 3-word objects and I wonder if it would be
reasonable to adapt the granule size accordingly.
The application requires 64-bit pointers and has no double-word alignment
requirements, so I would be keen on reducing the memory requirement.
I'm grateful for any advise and/or tips!
Best Regards,
Stefan.
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[Stefan Kral]

That is okay with me... At the moment the GC allocates memory in chunks of
16 bytes, so I lose 8 bytes everytime I allocate a 24 byte block.

Would lowering the granule size to 8 bytes likely cause any problems? (If
most of my allocations are small...)

Regards, Stefan.

Post by Bruce Hoult
The GC needs to store a pointer for a free list, so needs at least 8 bytes
per object. And aligned on most CPUs.

Post by Stefan Kral
Hello.
My app operates on a lot of 3-word objects and I wonder if it would be
reasonable to adapt the granule size accordingly.
The application requires 64-bit pointers and has no double-word alignment
requirements, so I would be keen on reducing the memory requirement.
I'm grateful for any advise and/or tips!
Best Regards,
Stefan.
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This message has been scanned for viruses and
dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is
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[Bruce Hoult]

It would be a definite bug if it didn't work with 8 bytes. Some support
data structures might use a bit more space.

Post by Stefan Kral
That is okay with me... At the moment the GC allocates memory in chunks of
16 bytes, so I lose 8 bytes everytime I allocate a 24 byte block.
Would lowering the granule size to 8 bytes likely cause any problems? (If
most of my allocations are small...)
Regards, Stefan.

Post by Bruce Hoult
The GC needs to store a pointer for a free list, so needs at least 8
bytes per object. And aligned on most CPUs.

Post by Stefan Kral
Hello.
My app operates on a lot of 3-word objects and I wonder if it would be
reasonable to adapt the granule size accordingly.
The application requires 64-bit pointers and has no double-word
alignment requirements, so I would be keen on reducing the memory
requirement.
I'm grateful for any advise and/or tips!
Best Regards,
Stefan.
--
This message has been scanned for viruses and
dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is
believed to be clean.

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