From dd215f646c72b4cf680097b13aeb8b0c589dceb2 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Llu=C3=ADs?= Date: Wed, 31 Aug 2011 20:31:38 +0200 Subject: trace: always use the "nop" backend on events with the "disable" keyword MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Any event with the keyword/property "disable" generates an empty trace event using the "nop" backend, regardless of the current backend. Signed-off-by: Lluís Vilanova --- docs/tracing.txt | 25 +++++++++++++++---------- 1 file changed, 15 insertions(+), 10 deletions(-) (limited to 'docs/tracing.txt') diff --git a/docs/tracing.txt b/docs/tracing.txt index 455da37969..85793cf173 100644 --- a/docs/tracing.txt +++ b/docs/tracing.txt @@ -12,15 +12,11 @@ for debugging, profiling, and observing execution. ./configure --trace-backend=simple make -2. Enable trace events you are interested in: - - $EDITOR trace-events # remove "disable" from events you want - -3. Run the virtual machine to produce a trace file: +2. Run the virtual machine to produce a trace file: qemu ... # your normal QEMU invocation -4. Pretty-print the binary trace file: +3. Pretty-print the binary trace file: ./simpletrace.py trace-events trace-* @@ -103,10 +99,11 @@ portability macros, ensure they are preceded and followed by double quotes: 4. Name trace events after their function. If there are multiple trace events in one function, append a unique distinguisher at the end of the name. -5. Declare trace events with the "disable" property. Some trace events can - produce a lot of output and users are typically only interested in a subset - of trace events. Marking trace events disabled by default saves the user - from having to manually disable noisy trace events. +5. If specific trace events are going to be called a huge number of times, this + might have a noticeable performance impact even when the trace events are + programmatically disabled. In this case you should declare the trace event + with the "disable" property, which will effectively disable it at compile + time (using the "nop" backend). == Generic interface and monitor commands == @@ -165,6 +162,9 @@ The "nop" backend generates empty trace event functions so that the compiler can optimize out trace events completely. This is the default and imposes no performance penalty. +Note that regardless of the selected trace backend, events with the "disable" +property will be generated with the "nop" backend. + === Stderr === The "stderr" backend sends trace events directly to standard error. This @@ -173,6 +173,11 @@ effectively turns trace events into debug printfs. This is the simplest backend and can be used together with existing code that uses DPRINTF(). +Note that with this backend trace events cannot be programmatically +enabled/disabled. Thus, in order to trim down the amount of output and the +performance impact of tracing, you might want to add the "disable" property in +the "trace-events" file for those events you are not interested in. + === Simpletrace === The "simple" backend supports common use cases and comes as part of the QEMU -- cgit v1.2.1