Unneeded displaced single stepping causes issues: Performance drop and range stepping killer

Raphael Zulliger zulliger@indel.ch
Wed Apr 24 06:58:00 GMT 2013


On 04/24/2013 06:53 AM, Tom Tromey wrote:
>>>>>> "Raphael" == Raphael Zulliger <zulliger@indel.ch> writes:
> Raphael> versus:
>
> Raphael>         (gdb) set displaced-stepping 1
> Raphael>         (gdb) si
> Raphael>         Sending packet: $m1508,4#9b...Packet received: 3c000058
> Raphael>         Sending packet: $m832d4,4#02...Packet received: 7c0802a6
> Raphael>         Sending packet: $X1508,0:#bc...Packet received:
> Raphael>         binary downloading NOT supported by target
> Raphael>         Sending packet: $M1508,4:7c0802a6#b0...Packet received: OK
> Raphael>         Sending packet: $g#67...Packet received:
> [...]
>
> I guess this is copying the instruction and stepping it?
Yes.
>   I wonder if we
> could put more information about displaced stepping into the remote
> protocol, to avoid the extra traffic.
Good point! Something like:
   vCont;d:p1.dca118
Would perfectly work for our system. (For our system it'd be required 
that GDB does the displaced step fixup, as we do not (yet) support this 
on the target side - others systems may even avoid that step).

Speaking in RSP, it could then look like this:
         (gdb) si
         Sending packet: $vCont;d:p1.dca118#83...Packet received: OK
           Notification received: 
Stop:T05thread:p1.dca118;0:00049cd0;1:00dcde40;2:00dca118;3:00dcdea4;4:00dcdea0;5:00000142;6:00dcde68;7:00000002;8:00000005;9:00dcdea4;a:00579e90;b:00ed4b34;c:00000000;d:f7ffffff;e:00000357;f:f1010000;10:00000000;11:fff71658;12:0068e738;13:001dd964;14:00000000;15:00000000;16:0069c120;17:00000000;18:00000037;19:00000000;1a:00000032;1b:00e6b2c0;1c:00087e48;1d:00ed12a0;1e:00ed1b68;1f:00dcde78;20:00000006347fd013;21:3ff199999999999a;22:3ff199999999999a;23:0000000000000000;24:0000000000000000;25:0000000000000000;26:0000000000000000;27:0000000000000000;28:0000000000000000;29:0000000000000000;2a:0000000000000000;2b:0000000000000000;2c:401900a3d70a3d71;2d:3ff199999999999a;2e:0000000000000000;2f:0000000000000000;30:0000000000000000;31:0000000000000000;32:0000000000000000;33:0000000000000000;34:0000000000000000;35:0000000000000000;36:0000000000000000;37:0000000000000000;38:0000000000000000;39:0000000000000000;3a:0000000000000000;3b:0000000000000000;3c:0000000000000000;3d:0000000000000000;3e:0000000000000000;3f:0000000000000000;46:efbeadde;40:0000150c;41:0008a030;42:20000004;43:00049cd0;44:00000000;45:00000000;
         Sending packet: $vStopped#55...Packet received: OK
         Sending packet: $P40=000832d8#ba...Packet received: OK

vs:

  (gdb) si
         Sending packet: $m1508,4#9b...Packet received: 3c000058
         Sending packet: $m832d4,4#02...Packet received: 7c0802a6
         Sending packet: $M1508,4:7c0802a6#b0...Packet received: OK
         Sending packet: $g#67...Packet received: 
00dcdea000dcde4000dca11800dcdea400dcdea00000014200dcde68000000020000000500dcdea400579e9000ed4b3400000000f7ffffff00000357f101000000000000fff716580068e738001dd96400000000000000000069c1200000000000000037000000000000003200e6b2c000087e4800ed12a000ed1b6800dcde7800000006347fd0133ff199999999999a3ff199999999999a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000401900a3d70a3d713ff199999999999a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000832d40008a0302000000400049cd00000000000000000efbeadde
         Sending packet: $P40=00001508#7f...Packet received: OK
         Sending packet: $vCont;s:p1.dca118#83...Packet received: OK
           Notification received: 
Stop:T05thread:p1.dca118;0:00049cd0;1:00dcde40;2:00dca118;3:00dcdea4;4:00dcdea0;5:00000142;6:00dcde68;7:00000002;8:00000005;9:00dcdea4;a:00579e90;b:00ed4b34;c:00000000;d:f7ffffff;e:00000357;f:f1010000;10:00000000;11:fff71658;12:0068e738;13:001dd964;14:00000000;15:00000000;16:0069c120;17:00000000;18:00000037;19:00000000;1a:00000032;1b:00e6b2c0;1c:00087e48;1d:00ed12a0;1e:00ed1b68;1f:00dcde78;20:00000006347fd013;21:3ff199999999999a;22:3ff199999999999a;23:0000000000000000;24:0000000000000000;25:0000000000000000;26:0000000000000000;27:0000000000000000;28:0000000000000000;29:0000000000000000;2a:0000000000000000;2b:0000000000000000;2c:401900a3d70a3d71;2d:3ff199999999999a;2e:0000000000000000;2f:0000000000000000;30:0000000000000000;31:0000000000000000;32:0000000000000000;33:0000000000000000;34:0000000000000000;35:0000000000000000;36:0000000000000000;37:0000000000000000;38:0000000000000000;39:0000000000000000;3a:0000000000000000;3b:0000000000000000;3c:0000000000000000;3d:0000000000000000;3e:0000000000000000;3f:0000000000000000;46:efbeadde;40:0000150c;41:0008a030;42:20000004;43:00049cd0;44:00000000;45:00000000;
         Sending packet: $vStopped#55...Packet received: OK
         Sending packet: $M1508,4:3c000058#78...Packet received: OK
         Sending packet: $P40=000832d8#ba...Packet received: OK

That would be quite an improvement!

While we are at it... I've got another note related to single-stepping: 
For our system, once a 'vCont;s:' returns with "OK", the single-step has 
already been performed. Maybe this is also the case for the majority of 
the systems out there? For systems like ours, we could get rid of the 
stop notification, by returning the content of the stop reply (sig 
number & registers) right in the vCont reply. (And, of course, the same 
would be true for range stepping.) That would again reduce the 
communication to something like:

         (gdb) si
         Sending packet: $vCont;d:p1.dca118#83...Packet received: 
T05thread:p1.dca118;0:00049cd0;1:00dcde40;2:00dca118;3:00dcdea4;4:00dcdea0;5:00000142;6:00dcde68;7:00000002;8:00000005;9:00dcdea4;a:00579e90;b:00ed4b34;c:00000000;d:f7ffffff;e:00000357;f:f1010000;10:00000000;11:fff71658;12:0068e738;13:001dd964;14:00000000;15:00000000;16:0069c120;17:00000000;18:00000037;19:00000000;1a:00000032;1b:00e6b2c0;1c:00087e48;1d:00ed12a0;1e:00ed1b68;1f:00dcde78;20:00000006347fd013;21:3ff199999999999a;22:3ff199999999999a;23:0000000000000000;24:0000000000000000;25:0000000000000000;26:0000000000000000;27:0000000000000000;28:0000000000000000;29:0000000000000000;2a:0000000000000000;2b:0000000000000000;2c:401900a3d70a3d71;2d:3ff199999999999a;2e:0000000000000000;2f:0000000000000000;30:0000000000000000;31:0000000000000000;32:0000000000000000;33:0000000000000000;34:0000000000000000;35:0000000000000000;36:0000000000000000;37:0000000000000000;38:0000000000000000;39:0000000000000000;3a:0000000000000000;3b:0000000000000000;3c:0000000000000000;3d:0000000000000000;3e:0000000000000000;3f:0000000000000000;46:efbeadde;40:0000150c;41:0008a030;42:20000004;43:00049cd0;44:00000000;45:00000000;
         Sending packet: $P40=000832d8#ba...Packet received: OK

>
> Tom
>




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