NAME¶
ttrace - Trace-based interpreter initialization
SYNOPSIS¶
package require
Tcl 8.4
package require
Thread ?2.6?
ttrace::eval arg ?arg ...?
ttrace::enable
ttrace::disable
ttrace::cleanup
ttrace::update ?epoch?
ttrace::getscript
ttrace::atenable cmd arglist body
ttrace::atdisable cmd arglist body
ttrace::addtrace cmd arglist body
ttrace::addscript name body
ttrace::addresolver cmd arglist body
ttrace::addcleanup body
ttrace::addentry cmd var val
ttrace::getentry cmd var
ttrace::getentries cmd ?pattern?
ttrace::delentry cmd
ttrace::preload cmd
DESCRIPTION¶
This package creates a framework for on-demand replication of the interpreter
state accross threads in an multithreading application. It relies on the
mechanics of Tcl command tracing and the Tcl
unknown command and
mechanism.
The package requires Tcl threading extension but can be alternatively used
stand-alone within the AOLserver, a scalable webserver from America Online.
In a nutshell, a short sample illustrating the usage of the ttrace with the Tcl
threading extension:
% package require Ttrace
2.6.5
% set t1 [thread::create {package require Ttrace; thread::wait}]
tid0x1802800
% ttrace::eval {proc test args {return test-[thread::id]}}
% thread::send $t1 test
test-tid0x1802800
% set t2 [thread::create {package require Ttrace; thread::wait}]
tid0x1804000
% thread::send $t2 test
test-tid0x1804000
As seen from above, the
ttrace::eval and
ttrace::update commands
are used to create a thread-wide definition of a simple Tcl procedure and
replicate that definition to all, already existing or later created, threads.
USER COMMANDS¶
This section describes user-level commands. Those commands can be used by script
writers to control the execution of the tracing framework.
- ttrace::eval arg ?arg ...?
- This command concatenates given arguments and evaluates the
resulting Tcl command with trace framework enabled. If the command
execution was ok, it takes necessary steps to automatically propagate the
trace epoch change to all threads in the application. For AOLserver, only
newly created threads actually receive the epoch change. For the Tcl
threading extension, all threads created by the extension are
automatically updated. If the command execution resulted in Tcl error, no
state propagation takes place.
This is the most important user-level command of the package as it wraps
most of the commands described below. This greatly simplifies things,
because user need to learn just this (one) command in order to effectively
use the package. Other commands, as desribed below, are included mostly
for the sake of completeness.
- ttrace::enable
- Activates all registered callbacks in the framework and
starts a new trace epoch. The trace epoch encapsulates all changes done to
the interpreter during the time traces are activated.
- ttrace::disable
- Deactivates all registered callbacks in the framework and
closes the current trace epoch.
- ttrace::cleanup
- Used to clean-up all on-demand loaded resources in the
interpreter. It effectively brings Tcl interpreter to its pristine
state.
- ttrace::update ?epoch?
- Used to refresh the state of the interpreter to match the
optional trace ?epoch?. If the optional ?epoch? is not given, it takes the
most recent trace epoch.
- ttrace::getscript
- Returns a synthetized Tcl script which may be sourced in
any interpreter. This script sets the stage for the Tcl unknown
command so it can load traced resources from the in-memory database.
Normally, this command is automatically invoked by other higher-level
commands like ttrace::eval and ttrace::update.
CALLBACK COMMANDS¶
A word upfront: the package already includes callbacks for tracing following Tcl
commands:
proc,
namespace,
variable,
load, and
rename. Additionaly, a set of callbacks for tracing resources (object,
clasess) for the XOTcl v1.3.8+, an OO-extension to Tcl, is also provided. This
gives a solid base for solving most of the real-life needs and serves as an
example for people wanting to customize the package to cover their specific
needs.
Below, you can find commands for registering callbacks in the framework and for
writing callback scripts. These callbacks are invoked by the framework in
order to gather interpreter state changes, build in-memory database, perform
custom-cleanups and various other tasks.
- ttrace::atenable cmd arglist
body
- Registers Tcl callback to be activated at
ttrace::enable. Registered callbacks are activated on FIFO basis.
The callback definition includes the name of the callback, cmd, a
list of callback arguments, arglist and the body of the
callback. Effectively, this actually resembles the call interface of the
standard Tcl proc command.
- ttrace::atdisable cmd arglist
body
- Registers Tcl callback to be activated at
ttrace::disable. Registered callbacks are activated on FIFO basis.
The callback definition includes the name of the callback, cmd, a
list of callback arguments, arglist and the body of the
callback. Effectively, this actually resembles the call interface of the
standard Tcl proc command.
- ttrace::addtrace cmd arglist
body
- Registers Tcl callback to be activated for tracing the Tcl
cmd command. The callback definition includes the name of the Tcl
command to trace, cmd, a list of callback arguments, arglist
and the body of the callback. Effectively, this actually resembles
the call interface of the standard Tcl proc command.
- ttrace::addscript name body
- Registers Tcl callback to be activated for building a Tcl
script to be passed to other interpreters. This script is used to set the
stage for the Tcl unknown command. Registered callbacks are
activated on FIFO basis. The callback definition includes the name of the
callback, name and the body of the callback.
- ttrace::addresolver cmd arglist
body
- Registers Tcl callback to be activated by the overloaded
Tcl unknown command. Registered callbacks are activated on FIFO
basis. This callback is used to resolve the resource and load the resource
in the current interpreter.
- ttrace::addcleanup body
- Registers Tcl callback to be activated by the
trace::cleanup. Registered callbacks are activated on FIFO
basis.
- ttrace::addentry cmd var
val
- Adds one entry to the named in-memory database.
- ttrace::getentry cmd var
- Returns the value of the entry from the named in-memory
database.
- ttrace::getentries cmd ?pattern?
- Returns names of all entries from the named in-memory
database.
- ttrace::delentry cmd
- Deletes an entry from the named in-memory database.
- ttrace::preload cmd
- Registers the Tcl command to be loaded in the interpreter.
Commands registered this way will always be the part of the interpreter
and not be on-demand loaded by the Tcl unknown command.
DISCUSSION¶
Common introspective state-replication approaches use a custom Tcl script to
introspect the running interpreter and synthesize another Tcl script to
replicate this state in some other interpreter. This package, on the contrary,
uses Tcl command traces. Command traces are registered on selected Tcl
commands, like
proc,
namespace,
load and other standard
(and/or user-defined) Tcl commands. When activated, those traces build an
in-memory database of created resources. This database is used as a resource
repository for the (overloaded) Tcl
unknown command which creates the
requested resource in the interpreter on demand. This way, users can update
just one interpreter (master) in one thread and replicate that interpreter
state (or part of it) to other threads/interpreters in the process.
Immediate benefit of such approach is the much smaller memory footprint of the
application and much faster thread creation. By not actually loading all
necessary procedures (and other resources) in every thread at the thread
initialization time, but by deffering this to the time the resource is
actually referenced, significant improvements in both memory consumption and
thread initialization time can be achieved. Some tests have shown that memory
footprint of an multithreading Tcl application went down more than three times
and thread startup time was reduced for about 50 times. Note that your mileage
may vary. Other benefits include much finer control about what (and when) gets
replicated from the master to other Tcl thread/interpreters.
SEE ALSO¶
thread, tpool, tsv
KEYWORDS¶
command tracing, introspection