I've completed some rudimentary test code. It is designed to compile both in user & kernel space but only currently compiles in userland. I ran this on 9 different versions of gcc, all on x86_64 with CFLAGS="-O2 -g3 -pipe -march=k8". The below summary data shows the % increase in time consumed (decrease in performance) using my generic red-black trees over hand-coded functions for insertion.
gcc ver % decrease in speed 4.7.1 -5.39% 4.6.2 2.60% 4.5.3 18.07% 4.4.6 20.52% 4.3.6 13.53% 4.2.4 11.84% 4.1.2 16.36% 4.0.4 35.70% 3.4.6 47.28% I don't understand why the generic code ran faster than hand-coded on gcc 4.7.1 as I haven't examined the assembly output yet. However, I'm pretty certain I understand why it was 2% slower on 4.6. This has to do with an optimization flaw. In the hand-coded insert/find functions, I used the "if (a->key > b->key) .. else if (a->key < b->key)" construct, where as the generic code calculates a diff and compares that against zero and 4.6.2 is adding an unnecessary cmp instruction: 3f2: 8b 48 18 mov 0x18(%rax),%ecx 3f5: 8b 7a 18 mov 0x18(%rdx),%edi 3f8: 48 29 f9 sub %rdi,%rcx 3fb: 48 83 f9 00 cmp $0x0,%rcx 3ff: 7f df jg 3e0 <grbtest_insert+0xb0> 401: 0f 84 c9 00 00 00 je 4d0 <grbtest_insert+0x1a0> The test configuration was for a tree that tracks both leftmost, rightmost and count and uses unique keys where the insert function replaces an existing object. These tests weren't ideal. While I allocated 4096 objects (32 bytes each) to stick in my tree, I used 0xff for a key mask, so only 256 object would be in the tree at once and I didn't notice this until I was most of the way through the tests. My intention was to run the test with a data set small enough to fit into the L3 cache, so as to reduce overhead from memory access and isolate the actual differences in the algorithm. When I get this all cleaned up, I'll release another patch set with the test code added. (This also automates correctness tests for find, insert, find_near and insert_near). Also, I'm going to experiment with branch prediction to see if I can squeeze a little more performance out of the older compilers. Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
