QFS and ARM 32bit platforms #213
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@valerioa Thanks! I was just struggling with this one myself. |
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While trying to run QFS on a cluster of ARMv7 boards (namely big.LITTLE cortex-a15.cortex-a7 ODROID XU4 board), I've discovered an issue that exists in running QFS on armv7 32 CPUs. The issue cannot be fixed by the QFS team, but, I'm recording it here for future gogglers, who might run into the same problem and might be looking for a solution. Feel free to close this issue once you've read it.
The issue is that the unmodified github source, compiled with a recent gcc on an ARM 32bit CPU will lead to a non working metaserver:
Vanilla compilation with gcc, will lead to a metaserver that, after correctly creating an empty filesystem,
will never starts, complaining of a missing root directory, which is actually there
It took me quite an amount of digging through source code, in order to find out that the problem was caused here, in src/cc/meta/meta.h:
This function calculates hash codes for directory and file names. Hash codes are used to find files and directories in the meta tree. In the example above, the root directory "/" is not found because
nameHash()returns the wrong hash code.Now, the data type returned by the hashing function
f(name)is an unsigned 32 bit quantity that is promoted to signed 64 bit quantity (the data type ofKeyDatais int64_t) and eventually left shifted by 4 bits.Here's the rub: gcc 4.9 (maybe earlier than that) through 6.3 has a bug. QFS is compiled with the default cmake flags of "-O2 -DNDEBUG". In trying to optimize this line:
gcc fuses the two operations (promotion to a larger int type and left shift) in a single group of assembler instructions, for speed. But, there is a bug that will insert the wrong armv7 asm instruction (an arithmetic right shift, instead of a left shift). This bug has been there for quite some time. I guess the c/c++ construct of having a promotion to a larger integer + left shift (close enough to be optimized by gcc) must not be very common in source code, because the bug has not been reported or observed for at least two years.
I've duly reported the bug to the gcc team.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79121
It has already been fixed for gcc 7.0 (the bug applies to arm 32 and mips when promoting from 32 to 64 bit quantity, and to arm64 when promoting from 64 to 128)
I don't know if and when it will back-ported to gcc 6.3 and earlier.
There are three workarounds for those who want to run QFS on arm 32 bit
Compile everything without any optimization. gcc will create the correct asm code without optimization
Compile with clang++/llvm
split promotion to larger int and left shift and add the "volatile" keyword to the KeyData assignment; vanilla compile without further editing (in src/cc/meta/meta.h)
volatilewill instruct gcc not to make the KeyData assignment part of any optimization, thus promotion and left shift will not be fused into the same cluster of asm instructions - no bug triggered.The last one is by far the simplest solution.
Future arm 32 googlers, you're welcome.
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