Saturday, May 9, 2009

threading, STL, boost - IV questions

Read-guard, write-guard ?

Q: how to test if current thread already holds the lock?
AA: Boost Non-member function get_id() Returns An instance of boost::thread::id class that represents that currently executing thread.
AA: pthread_self() is a C function (=> non-method) that returns the thread id of the current thread.
AA: see the pthreads recursive mutex note below.
A: using this thread::id object a lock can "remember" which thread is holding it.

Q: Is scoped_lock reentrant by default? Can you make it reentrant?
AA: With a boost recursive mutex, a single thread may lock the same mutex several times and must unlock the mutex the **same-number-of-times**.

AA: pthreads supports it. http://www.opengroup.org/onlinepubs/009695399/functions/pthread_mutex_lock.html says -- If the mutex type is PTHREAD_MUTEX_RECURSIVE and the mutex is currently owned by the calling thread, the mutex lock count shall be incremented by one and the pthread_mutex_trylock() function shall immediately return success.

Q: Boost supports try_lock()?
AA: yes

Q: Boost supports lockInterruptibly()?
A: probably not, but there's timed_lock

Q: Boost supports reader/writer locks?
AA: read/write lock in the form of boost::shared_mutex

Q: difference between mutex and condition var?

Q: can you write code to implement "recursive" locking with reader/writer threads?

Q: what types of smart pointers did you use and when. What are the differences?

Q: what other boost lib?
%%A: I briefly used boost bind. I believe STL binders are imperfect.

Q1: Say I have a small db table of 10 columns x 100 rows. Keys are
non-unique. To cache it we want to use STL containers. What container?
%%A: multimap or list. Not hashed map. I may start with a vector, for simplicity.

Q1a: search?
%A: for a sorted container, just lookup using this->find(). For list, iterate using generic find()

Q1c: what if I have a list of keys to search?
%%A: is there an "intersect()" algorithm? If none, then I would write my nested iteration. Loop through the target keys, and find() on each.
A: for_each()?

Q1e: how do you hold the 10 col?
%%A: each object in container will have 10 fields. They could be 10 custom data classes or strings, ints, floats. Probably 10 pointers for maximum flexibility.

Q1h: what if I have other tables to cache too?
%%A: parametrize the CacheService class. CacheService class will be a wrapper of the vector. There will be other fields beside the vector.

Q1m: how about the data class? Say you have a position table and account table to cache
%%A: either inheritance or template. If they have nothing in common, then maintain 2 unrelated classes.

Q2: if I want a custom class as a map key?
%%A: must overload the less-than operator

Q2c: what if I can't change the source code?
%%A: derive from binary_function to get a functor, and pass it to map
constructor as the optional type param. However, the new functor only
has "operator()" overloaded? I think it will still work, as the
function doesn't have to be named "less()".
AA: Item 42 [[eff STL]]

%%Q2g: is operator less-than a friend func or a method?
AA: can be a non-friend or a friend class/func.

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