* gprof
@ 2005-09-15 10:54 Michael Trimarchi
2005-09-15 12:31 ` gprof Ken Raeburn
0 siblings, 1 reply; 5+ messages in thread
From: Michael Trimarchi @ 2005-09-15 10:54 UTC (permalink / raw)
To: binutils
Hi all,
I'd like to profile multithread application using gprof, but I cannot
profile thread !!! why?
Is the ITIMER signal problem?
regards
Michael
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: gprof
2005-09-15 10:54 gprof Michael Trimarchi
@ 2005-09-15 12:31 ` Ken Raeburn
2005-09-20 14:13 ` gprof Michael Trimarchi
0 siblings, 1 reply; 5+ messages in thread
From: Ken Raeburn @ 2005-09-15 12:31 UTC (permalink / raw)
To: Michael Trimarchi; +Cc: binutils
On Sep 15, 2005, at 04:45, Michael Trimarchi wrote:
> Hi all,
> I'd like to profile multithread application using gprof, but I
> cannot profile thread !!! why?
> Is the ITIMER signal problem?
> regards
> Michael
I've tried this before... yes, depending on the system, you may need
to play some games after thread creation to start profiling in the
newly created thread, such as explicitly scheduling an interrupt
timer. But also, if you care about the accuracy of the results, you
may need to modify the C runtime support code for profiling, which
typically updates the per-function data in a manner that is not
thread-safe.
Unfortunately, I don't have any code to show you for either issue,
right now...
Ken
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: gprof
2005-09-15 12:31 ` gprof Ken Raeburn
@ 2005-09-20 14:13 ` Michael Trimarchi
2005-09-20 18:12 ` gprof Ken Raeburn
0 siblings, 1 reply; 5+ messages in thread
From: Michael Trimarchi @ 2005-09-20 14:13 UTC (permalink / raw)
To: Ken Raeburn; +Cc: binutils
Ken Raeburn wrote:
> On Sep 15, 2005, at 04:45, Michael Trimarchi wrote:
>
>> Hi all,
>> I'd like to profile multithread application using gprof, but I
>> cannot profile thread !!! why?
>> Is the ITIMER signal problem?
>> regards
>> Michael
>
>
> I've tried this before... yes, depending on the system, you may need
> to play some games after thread creation to start profiling in the
> newly created thread, such as explicitly scheduling an interrupt
> timer. But also, if you care about the accuracy of the results, you
> may need to modify the C runtime support code for profiling, which
> typically updates the per-function data in a manner that is not
> thread-safe.
>
> Unfortunately, I don't have any code to show you for either issue,
> right now...
>
> Ken
>But also, if you care about the accuracy of the results, you may need
to modify the C runtime support code for >profiling, which typically
updates the per-function data in a manner that is not thread-safe.
may you explain more precisely this point?
Regards
Michael
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: gprof
2005-09-20 14:13 ` gprof Michael Trimarchi
@ 2005-09-20 18:12 ` Ken Raeburn
2005-09-27 10:59 ` gprof Michael Trimarchi
0 siblings, 1 reply; 5+ messages in thread
From: Ken Raeburn @ 2005-09-20 18:12 UTC (permalink / raw)
To: Michael Trimarchi; +Cc: binutils
On Sep 20, 2005, at 06:31, Michael Trimarchi wrote:
> >But also, if you care about the accuracy of the results, you may
> need to modify the C runtime support code for >profiling, which
> typically updates the per-function data in a manner that is not
> thread-safe.
> may you explain more precisely this point?
> Regards
> Michael
With basic profiling, the runtime support code keeps track of how
often the CPU program counter is in a given range of values, with
fairly fine granularity. Later this table is dumped out, and (g)prof
interprets it in combination with symbol information from the
executable. For example, addresses XXX through YYY correspond to
function A, and so many ticks at a certain frequency were counted
with the PC in that range, so here's the amount of time the program
spent in that function. But that may be inaccurate in multithreaded
programs if the counter is implemented as read counter value N,
increment value, another thread runs for a bit and changes the
counter, store counter value N+1; you've just lost the change made by
the other thread.
For graph profiling, you also need data recorded on entry to a
function indicating where the function was called from, and how many
times it was called from each call site. Since you could have
arbitrarily many such call sites, this is likely to involve dynamic
memory allocation, walking through some data structures, etc. If
it's not done just right, it might even result in crashes in
multiprocessor, multithreaded situations, if you're really unlucky
with the timing.
I've never gotten around to modifying the support code to try to make
it thread-safe. Using mutex locks would be the obvious approach, but
probably kind of expensive compared to some of the atomic operations
a few processors have available, or "store if another cpu or thread
hasn't stored here, and set condition codes to tell me", etc.
You might just get kind of lucky with the existing support code,
though, your program might not crash, and the numbers might even be
vaguely accurate...
Ken
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: gprof
2005-09-20 18:12 ` gprof Ken Raeburn
@ 2005-09-27 10:59 ` Michael Trimarchi
0 siblings, 0 replies; 5+ messages in thread
From: Michael Trimarchi @ 2005-09-27 10:59 UTC (permalink / raw)
To: Ken Raeburn; +Cc: binutils
Ken Raeburn wrote:
> On Sep 20, 2005, at 06:31, Michael Trimarchi wrote:
>
>> >But also, if you care about the accuracy of the results, you may
>> need to modify the C runtime support code for >profiling, which
>> typically updates the per-function data in a manner that is not
>> thread-safe.
>> may you explain more precisely this point?
>> Regards
>> Michael
>
>
> With basic profiling, the runtime support code keeps track of how
> often the CPU program counter is in a given range of values, with
> fairly fine granularity. Later this table is dumped out, and (g)prof
> interprets it in combination with symbol information from the
> executable. For example, addresses XXX through YYY correspond to
> function A, and so many ticks at a certain frequency were counted
> with the PC in that range, so here's the amount of time the program
> spent in that function. But that may be inaccurate in multithreaded
> programs if the counter is implemented as read counter value N,
> increment value, another thread runs for a bit and changes the
> counter, store counter value N+1; you've just lost the change made by
> the other thread.
>
> For graph profiling, you also need data recorded on entry to a
> function indicating where the function was called from, and how many
> times it was called from each call site. Since you could have
> arbitrarily many such call sites, this is likely to involve dynamic
> memory allocation, walking through some data structures, etc. If
> it's not done just right, it might even result in crashes in
> multiprocessor, multithreaded situations, if you're really unlucky
> with the timing.
>
> I've never gotten around to modifying the support code to try to make
> it thread-safe. Using mutex locks would be the obvious approach, but
> probably kind of expensive compared to some of the atomic operations
> a few processors have available, or "store if another cpu or thread
> hasn't stored here, and set condition codes to tell me", etc.
>
> You might just get kind of lucky with the existing support code,
> though, your program might not crash, and the numbers might even be
> vaguely accurate...
>
> Ken
>
For example, to compute the time spent inside a function foo()
(starting at address X and ending at address Y) the system sample each
tick looking if the program counter is between X and Y. That may be
inaccurate in multithreaded programs if the accounting of a
counter C is implemented as the following set of instructions:
read counter value C
C=C+1
store C
If another thread runs for a bit interrupting the thread between 2 and 3,
and that thread changes the counter, then the value stored by the
interrupted
thread is N+1, losing an increment. Is it ok?
Regards Michael
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2005-09-27 9:25 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2005-09-15 10:54 gprof Michael Trimarchi
2005-09-15 12:31 ` gprof Ken Raeburn
2005-09-20 14:13 ` gprof Michael Trimarchi
2005-09-20 18:12 ` gprof Ken Raeburn
2005-09-27 10:59 ` gprof Michael Trimarchi
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).