Bug 151817 – bmalloc: extra large allocations could be more efficient

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Michael Saboff]

BMalloc aligns extra large allocations to start on super chunk size aligned addresses.  It does this by increasing the allocation size to be at least two super chunks and then deallocating the front and back memory to get the start address to be super chunk aligned.  Given that the super chunk size is 4MB, we always ask the OS for 8MB for allocations 2MB or more and return the unused.  It makes sense to reduce the super chunk size to 2MB.

Secondly, when realloc is called on an extra large allocated piece of memory, we always allocate new memory and memcpy from the old.  There is a good possibility that we can get the memory for the increase from adjacently allocated memory, thus saving having both region allocated at once and the memcpy.

[Michael Saboff]

<rdar://problem/23639897>

[Michael Saboff]

Created attachment 266545 [details]
Patch

[Geoffrey Garen]

Comment on attachment 266545 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=266545&action=review

r=me

> Source/bmalloc/bmalloc/VMAllocate.h:36
> +#include <wtf/DataLog.h>

Revert.

> Source/bmalloc/bmalloc/VMAllocate.h:92
> +    if (vmOldSize >= vmNewSize)
> +        return true;

I think this should be an ASSERT instead of a check.

This function doesn't truly honor a shrink in size, since it doesn't shrink the allocation. So it should not pretend to succeed.

[Michael Saboff]

Created attachment 266552 [details]
Removed debugging include to fix build

[Geoffrey Garen]

Comment on attachment 266552 [details]
Removed debugging include to fix build

View in context: https://bugs.webkit.org/attachment.cgi?id=266552&action=review

r=me

> Source/bmalloc/bmalloc/VMAllocate.h:91
> +    if (vmOldSize >= vmNewSize)
> +        return true;

Same comment about ASSERT here.

[Michael Saboff]

Created attachment 266553 [details]
Patch for landing

Didn't see the comments when I posted the last patch.  Changed the "if" into an assert as suggested.

[WebKit Commit Bot]

Comment on attachment 266553 [details]
Patch for landing

Clearing flags on attachment: 266553

Committed r193373: <http://trac.webkit.org/changeset/193373>

[WebKit Commit Bot]

All reviewed patches have been landed.  Closing bug.