Download Text - Barrelfish

2. Background
2.1.3
Inter Dispatcher Communication & Flounder
In Barrelfish, a dispatcher is a unit of kernel scheduling and is responsible for the management of the kernel’s threads. Communication between
different dispatchers (so called Inter Dispatcher Communication or IDC) is
performed over bidirectional communication channels. A channel is defined
by Flounder, an interface definition language. A Flounder interface is used
to describe the messages exchanged over a channel. At compile time the
interface is translated into C code, which allows applications to use stubs to
send messages. A server can export an interface by sending a message to the
monitor. The monitor then assigns a unique interface reference (iref) to the
interface. Clients can use this iref to identify the service and initiate a connection. Barrelfish has a name service called Chips. Chips is used to store
and look-up irefs based on a name chosen by the server. There are a number
of back-ends for message passing in Barrelfish. The one chosen at runtime
depends on the underlying hardware and the location of the sender and receiver. For example, on x86 hardware there are two implementations: local
message passing (LMP) using hardware registers and user message passing
(UMP) using shared memory. Whereas LMP is used for IDC between two
dispatchers on the same core, IDC between different cores uses UMP. Which
mechanism is used is determined during connection set-up.
2.1.4
Capabilities
In order to keep track of global resources, Barrelfish uses capabilities [14] to
manage ownership and type of a resource. An area in Barrelfish that uses
capabilities is the memory system: In case a program needs access to RAM
it can request a capability for a given region of memory. The capability is
then used for all calls to the kernel to invoke operations on that memory
region. In contrast to access control lists (ACL) it is possible to provide
fine-grained access to a specific set of resources for individual user-space
programs. User-space programs do not have direct access to capabilities but
instead use capability references. This allows them to pass these references
around and delegate permissions to others.
2.1.5
THC
THC [11] is a set of language constructs for composable asynchronous I/O.
THC is integrated in the message passing system in Barrelfish. THC’s benefit
is that it avoids "stack-ripped" code: Regular Flounder bindings require to
specify callbacks to handle incoming messages. Plus, sending a message
can fail if the communication channel is currently busy. This usually forces
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