Jens Axboe writes
> I don't really see the patch doing what you describe - the indirect
> function calls are the same.
For example on Pentium4 in the function elv_next_request() the line
struct request *rq =
q->elevator->ops->elevator_next_req_fn(q);
before patch had required 11% of function running time as oprofile
reports
%
26 0.0457 :c0270ecb: mov 0xc(%edi),%eax
3455 6.0670 :c0270ece: mov (%eax),%eax
2848 5.0011 :c0270ed0: mov %edi,(%esp)
1538 2.7008 :c0270ed3: call *0xc(%eax)
A patch which would delete all indirect calls was tryed
struct request *rq = q->elevator_cp.ops.elevator_next_req_fn(q);
9 0.0224 :c0270eea: mov %edi,(%esp)
3814 9.4793 :c0270eed: call *0x18(%edi)
But additional memory would be needed for 'ops' in each queue. The
intermediate (proposed) patch has the same timing effect but saves some
memory:
struct request *rq =
q->elevator_cp.ops->elevator_next_req_fn(q);
drivers/block/elevator.c:351
ffffffff802a8b97: 49 8b 44 24 18 mov 0x18(%r12),%rax
ffffffff802a8b9c: 4c 89 e7 mov %r12,%rdi
ffffffff802a8b9f: ff 50 18 callq *0x18(%rax)
For Itanium the difference is huge:
Before patch:
drivers/block/elevator.c:351
a0000001002cbb60: 0d f0 00 4c 18 10 [MFI] ld8 r30=[r38]
a0000001002cbb66: 00 00 00 02 00 c0 nop.f 0x0
a0000001002cbb6c: 05 00 01 84 mov r46=r32;;
a0000001002cbb70: 0b e8 00 3c 18 10 [MMI] ld8 r29=[r30];;
a0000001002cbb76: c0 c1 74 00 42 00 adds r28=24,r29
a0000001002cbb7c: 00 00 04 00 nop.i 0x0;;
a0000001002cbb80: 0b d0 00 38 18 10 [MMI] ld8 r26=[r28];;
a0000001002cbb86: b0 41 68 30 28 00 ld8 r27=[r26],8
a0000001002cbb8c: 00 00 04 00 nop.i 0x0;;
a0000001002cbb90: 11 08 00 34 18 10 [MIB] ld8 r1=[r26]
a0000001002cbb96: 70 d8 04 80 03 00 mov b7=r27
a0000001002cbb9c: 78 00 80 10
br.call.sptk.many
After patching there is no object code for considered line. It
is scattered mixed with other source code lines.
Leonid
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