@@ -4,6 +4,36 @@ A collection of stuff that we noticed during development.
Useful later on two write a project report and to go back
in time to find out why certain decisions where made.
## 02.08.2019 - Ideas for sleeping threads when no work is available
A new requirement discovered during theoretical analysis is to
sleep threads when there is no work left to do. This, in
contrast to classic Arora, Blumofe and Plaxton (ABP) yielding
mechanism, we wan threads without work to allow LOWER PRIORITY
processes to take over. Simply yielding is not sufficient,
as it will not consider lower priority tasks.
Therefore, we need to actually sleep/wait in the time without any
work. This sleep is a conditional sleep: once work is available,
the thread should wake up and perform it to make sure high priority
processes finish in time. Usually one would use condition variables
for this kind of wait, but a posix condition variable requires to
hold a mutex to be race free, which we would like to avoid to keep
the whole system lock free (and thus keep our progress guarantee).
To keep a lock free mutex we found the `futex` linux syscall.
This is platform dependent on Linux, but as a system call is required
for any kind of sleep functionality we think it is ok to go for
OS specific primitives to implement this (a fallback for locking,
posix mutex + condition variables is always possible).
Some information can on `futex` can be found [in this bolg post](https://gustedt.wordpress.com/2011/01/28/linux-futexes-non-blocking-integer-valued-condition-variables/)
and [in the liunx man-pages](http://man7.org/linux/man-pages/man2/futex.2.html).
Basically, it allows to conditionally sleep a thread based on an
atomic compare and exchange operation on a 32 bit word,
which is exactly what we need for our lock free sleep based on an
indicator variable of available work.
## 31.07.2019 - First benchmark of implementation
We chose the implementation that allocates one invocation instance
