http://llvm.org/bugs/show_bug.cgi?id=3724
Summary: JIT Memory Manager causes false error with any RWX
memory region fragmentation
Product: libraries
Version: trunk
Platform: PC
OS/Version: Linux
Status: NEW
Keywords: quality-of-implementation
Severity: normal
Priority: P2
Component: Target-Independent JIT
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
The JIT memory manager always returns the head of the FreeMemoryList in
response to a block allocation request from JITEmitter
(JITMemoryManager.cpp:303)
This can cause a "JIT: Ran out of space for generated machine code" error
whenever there's any fragmentation in the MemoryManger RWX region, even if
there's a huge amount of free space left.
Consider a multi-threaded application, where Thread A allocates a 100-byte
block at the beginning of the RWX region, then Thread B allocates a 100-byte
block directly after A's block. Thread A then frees its machine code block.
This would leave:
| <--100B Free--> | <--100B Alloc(Thread B)--> | <--15+MB Free Space--> |
^
Head of free list
Now, whenever anything else tries to allocate a JIT region, the Memory Manager
will always return the first (empty) 100B block. If this isn't big enough,
however, the current behavior of the JITEmitter is to abort with a "JIT: Ran
out of space for generated machine code" error (JITEmitter.cpp:893). I know
there's a //FIXME in there, but even when the JITEmitter requests another
allocation, the JIT MemoryManager will still return the head block.
Proposed Fix: Since the allocation size is unknown at alloc time, the JIT
MemoryManager should return the largest available block in the FreeMemoryList
on any allocation. I have a patch locally that implements this behavior and
corrects the problem. There is of course a performance penalty for this, but in
my application, in practice, the free list never grows beyond a handful of
entries despite being accessed asynchronously by many threads.
--
Configure bugmail: http://llvm.org/bugs/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.
_______________________________________________
LLVMbugs mailing list
[email protected]
http://lists.cs.uiuc.edu/mailman/listinfo/llvmbugs
