Slides on cross-domain call and Remote Procedure Call (RPC)

This classic paper is a good example of a microbenchmarking study. It also explains the RPC abstraction and serves as a case study of the nuts-and-bolts of I/O, and related performance issues. Or is it “just hacking”?

Request/reply messaging clientserver request reply compute

Messaging: examples and variations Details vary! – Supercomputing: MPI over fast interconnect – High-level messages (e.g., HTTP) over sockets and network communication – Microkernel / Mach / MacOS: high-speed local cross- domain messaging ports. (Also Windows/NT) – Android: binder, and per-thread message queues Common abstraction: “Remote Procedure Call” – RPC for clients/serves talking over a network. – For local processes it is often called cross-domain call or “Local Procedure Call” (LPC, in Windows).

Network File System (NFS) [ucla.edu] Remote Procedure Call (RPC) External Data Representation (XDR)

Cross-domain call: the basics A B Request: block A, wakeup B. Reply: block B, wakeup A. A: syscall to post a message to B (e.g., a message queue). Wait for reply. B: syscalls to receive an incoming message. Wait for request.

Cross-domain call: the basics A B A: syscall to post a message to B (e.g., a message queue). Wait for reply. B: syscalls to receive an incoming message. Wait for request. Copy data from A to B, or use a shared memory region. Transfer control through kernel: block A, wakeup B. Note: could use a socket, or fast IPC for processes on same host.

“Marshalling” (“serializing”) A B What if the data is a complex linked structure? Must “pack” it as a sequence of bytes into a message, and reconstitute it on the other side.

Concept: RPC Remote Procedure Call (RPC) is request/response interaction through a published API, using IPC messaging to cross an inter- process boundary. API stubs RPC is used in many standard Internet services. It is also the basis for component frameworks like DCOM, CORBA, and Android. Software is packaged into named “objects” or components. Components may publish interfaces and/or invoke published interfaces of other components. Components may execute in different processes and/or on different nodes. generated from an Interface Description Language (IDL) Establishing an RPC connection to a named remote interface is often called binding.

The classic picture Implementing RPC Birrell/Nelson 1984

RPC Execution In general, RPC enables request/response exchanges (e.g., by messaging over a network) that “looks like” a local procedure call. In Android, RPC allows flexible interaction among apps running in different processes, across the kernel boundary. How is this different from a local procedure call? How is it different from a system call?

RPC: Language integration

Stubs link with the client/server code to “hide” the boundary crossing. – They “marshal” args/results – i.e., translate to/from some standard network stream format – Also known as linearize, serialize – …or “flatten” – Propagate PL-level exceptions – Stubs are auto-generated from an Interface Description Language (IDL) file by a stub compiler tool at software build time, and linked in. – Client and server must agree on the protocol signatures in the IDL file.

Marshalling: a metaphor Android Architecture and Binder Dhinakaran Pandiyan Saketh Paranjape

Stubs RPC stubs are procedures linked into the client and server. – RPC stubs are similar to system call stubs, but they do more than just trap to the kernel. – The RPC stubs construct/deconstruct a message transmitted through a messaging system. – Binder is an example of such a messaging system, implemented as a Linux kernel plug-in module (a driver) and some user-space libraries. The stubs are generated by a tool that takes a description of the application’s RPC API written in an Interface Description Language. – Looks like any interface definition… – List of method names and argument/result types and signatures. – Stub code marshals arguments into request message, marshals results into a reply message.

Stubs and IDL This picture illustrates the stub generation and build process for an RPC system based on the C language (e.g., ONC or Sun RPC, used in NFS).

Another picture of RPC Implementing RPC Birrell/Nelson 1984

Threads and RPC [OpenGroup, late 1980s] Q: How do we manage these “call threads”? A: Create them as needed, and keep idle threads in a thread pool. When an RPC call arrives, wake up an idle thread from the pool to handle it. On the client, the client thread blocks until the server thread returns a response.

Thread pool: idealized Incoming request (event) queue worker loop Magic elastic worker pool Resize worker pool to match incoming request load: create/destroy workers as needed. dispatch idle workers Workers wait here for next request dispatch. (Workers are threads.) Handle one event, blocking as necessary. When handler is complete, return to worker pool. handler

Event/request queue Incoming event queue worker loop Handle one event, blocking as necessary. When handler is complete, return to worker pool. We can synchronize an event queue with a monitor: a mutex/CV pair. Protect the event queue data structure itself with the mutex. dispatch threads waiting on CV Workers wait on the CV for next event if the event queue is empty. Signal the CV when a new event arrives. This is a producer/consumer problem. handler

Some details How is incoming data delivered to the correct process? On the return, how does the Receiver know which thread to wake up? How does the wakeup happen? What if a request/reply is dropped in the net? What if a request/reply is duplicated? How does the client find the server? (binding) What if the server fails? How to go faster if client/server are on the same host? (“LRPC” or “LPC”)

Firefly vs. Web/HTTP etc. Firefly does not use TCP/IP. Instead, it has a custom packet protocol. Tradeoffs? But some of the basics of network communication are similar/identical. How is (say) HTTP different from RPC?

Networked services: big picture Internet “cloud” server hosts with server applications client applications NIC device kernel network software client host Data is sent on the network as messages called packets.

A simple, familiar example “GET /images/fish.gif HTTP/1.1” sd = socket(…); connect(sd, name); write(sd, request…); read(sd, reply…); close(sd); s = socket(…); bind(s, name); sd = accept(s); read(sd, request…); write(sd, reply…); close(sd); request reply client (initiator)server

End-to-end data transfer transmit packet to network interface move data from application to system buffer TCP/IP protocol compute checksum network driver sender deposit packet in host memory move data from system buffer to application TCP/IP protocol compare checksum network driver receiver DMA + interrupt buffer queues (mbufs, skbufs) buffer queues packet queues

Ports and packet demultiplexing Data is sent on the network in messages called packets addressed to a destination node and port. Kernel network stack demultiplexes incoming network traffic: choose process/socket to receive it based on destination port. Network adapter hardware aka, network interface controller (“NIC”) Incoming network packets Apps with open sockets

Wakeup from interrupt handler sleep ready queue interrupt trap or fault return to user mode wakeup sleep queue switch Example 1: NIC interrupt wakes thread to receive incoming packets. Example 2: disk interrupt wakes thread when disk I/O completes. Example 3: clock interrupt wakes thread after N ms have elapsed. Note: it isn’t actually the interrupt itself that wakes the thread, but the interrupt handler (software). The awakened thread must have registered for the wakeup before sleeping (e.g., by placing its TCB on some sleep queue for the event).

Process, kernel, and syscalls trap read() {…} write() {…} copyout copyin user buffers kernel process user space read() {…} syscall dispatch table I/O descriptor table syscall stub Return to user mode I/O objects

Firefly: shared buffers Performance of Firefly RPC Michaels Schroeder and Burrows

Binding Implementing RPC Birrell/Nelson 1984

Optimize for the common case Performance of Firefly RPC Michaels Schroeder and Burrows The slower path through the operating-system address space is used when the interrupt routine cannot find the appropriate RPC thread in the call table, when it encounters a lock conflict in the call table, or when it handles a non-RPC packet. Several of the structural features used to improve RPC performance collapse layers of abstraction. Programming a fast RPC is not for the squeamish.

Latency and throughput Performance of Firefly RPC Michaels Schroeder and Burrows

Marshalling overhead Performance of Firefly RPC Michaels Schroeder and Burrows

Steps and overhead Performance of Firefly RPC Michaels Schroeder and Burrows

Performance of Firefly RPC Michaels Schroeder and Burrows

Performance of Firefly RPC Michaels Schroeder and Burrows

Performance of Firefly RPC Michaels Schroeder and Burrows

Performance of Firefly RPC Michaels Schroeder and Burrows

ASPLOS 1991

Schroeder and Burrows suggest that tripling CPU speed would reduce SRC RPC latency for a small packet by about 50%, on the expectation that the 83% of the time not spent on the wire will decrease by a factor of 3. Looking at Table 3, however, we see that much of the RPC time goes to functions that may not benefit proportionally from modern architectures. ……The only real ‘computation” in RPC, in the traditional sense, is the checksum processing, and this in fact is memory-intensive and not compute- intensive; each checksum addition is paired with a load …. Thus, Ousterhout found in the Sprite operating system [Ousterhout et al. 88] that kernel-to-kernel null RPC time was reduced by only half when moving from a Sun-3/75 to a SPARCstation-l, even though integer performance increased by a factor of five [Ousterhout 90a].

Android: object-based RPC channels JVM+lib Linux kernel Activity Manager Service etc. Android services and libraries communicate by sending messages through shared-memory channels set up by binder. JVM+lib Android binder A client binds to a service. Bindings are reference-counted. Services register to advertise for clients. an add-on kernel driver for /dev/binder object RPC

Binder is a add-on driver module that runs in the kernel. Unix drivers can define arbitrary “I/O control” APIs invoked through the ioctl system call. The ioctl syscall was designed for device control, but it serves as a general mechanism to extend the kernel and syscall interface (“kitchen sink”). Kernel space

Binder: thread pool details “The system maintains a pool of transaction threads in each process that it runs in. These threads are used to dispatch all IPCs coming in from other processes. For example, when an IPC is made from process A to process B, the calling thread in A blocks in transact() as it sends the transaction to process B. The next available pool thread in B receives the incoming transaction, calls Binder.onTransact() on the target object, and replies with the result Parcel. Upon receiving its result, the thread in process A returns to allow its execution to continue. …” [ Note: in this setting, a “transaction” is just an RPC request/response exchange.

Stubs and Interface Description Language This picture illustrates the Android class structure for objects invoked over binder RPC. …including classes generated via Android’s IDL (AIDL).