Messages in this thread | | | Date | Thu, 23 Mar 2006 12:08:31 +0100 | From | Andreas Mohr <> | Subject | mm/hugetlb.c/alloc_fresh_huge_page(): slow division on NUMA |
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Hello all,
following up on my previous mail (subject "ring buffer indices: way too much modulo (division!) fiddling"), I switched my .config to a NUMA setup and found that on NUMA there indeed is an idiv opcode in the mm/hugetlb.o output:
138: e8 fc ff ff ff call 139 <alloc_fresh_huge_page+0x32> 13d: 8b 1d 10 00 00 00 mov 0x10,%ebx 143: 89 c6 mov %eax,%esi 145: 83 c3 01 add $0x1,%ebx 148: c7 44 24 04 10 00 00 movl $0x10,0x4(%esp) 14f: 00 150: c7 04 24 00 00 00 00 movl $0x0,(%esp) 157: e8 fc ff ff ff call 158 <alloc_fresh_huge_page+0x51> 15c: 89 c2 mov %eax,%edx 15e: 89 d8 mov %ebx,%eax 160: 89 d1 mov %edx,%ecx 162: 99 cltd 163: f7 f9 idiv %ecx 165: 85 f6 test %esi,%esi 167: 89 15 10 00 00 00 mov %edx,0x10 16d: 74 3a je 1a9 <alloc_fresh_huge_page+0xa2> 16f: b8 00 00 00 00 mov $0x0,%eax 174: e8 fc ff ff ff call 175 <alloc_fresh_huge_page+0x6e>
Changing the code to use:
/* nid = (nid + 1) % num_online_nodes(); */ nid++; if (nid >= num_online_nodes()) nid = 0;
results in:
139: e8 fc ff ff ff call 13a <alloc_fresh_huge_page+0x33> 13e: 83 05 10 00 00 00 01 addl $0x1,0x10 145: 89 c3 mov %eax,%ebx 147: c7 44 24 04 10 00 00 movl $0x10,0x4(%esp) 14e: 00 14f: c7 04 24 00 00 00 00 movl $0x0,(%esp) 156: e8 fc ff ff ff call 157 <alloc_fresh_huge_page+0x50> 15b: 39 05 10 00 00 00 cmp %eax,0x10 161: 7c 0a jl 16d <alloc_fresh_huge_page+0x66> 163: c7 05 10 00 00 00 00 movl $0x0,0x10 16a: 00 00 00 16d: 85 db test %ebx,%ebx 16f: 74 3a je 1ab <alloc_fresh_huge_page+0xa4> 171: b8 00 00 00 00 mov $0x0,%eax 176: e8 fc ff ff ff call 177 <alloc_fresh_huge_page+0x70>
avoiding the idiv slowness.
At this point I wanted to add a huge rant that while this is faster, we're now not thread-safe any more (I thought that the modulo increment was an atomic operation), but analyzing the above code it is obvious that both versions are not atomic, so sending a patch with this change should be fine I guess?
Andreas Mohr - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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