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
client server
request
compute
reply 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)
Remote Procedure Call (RPC) External Data Representa on (XDR) [ucla.edu] Cross-domain call: the basics
A B
A: syscall to post a B: syscalls to receive an message to B (e.g., a incoming message. message queue). Wait Wait for request. for reply.
Request: block A, wakeup B. Reply: block B, wakeup A. Cross-domain call: the basics
Copy data from A to B, or use a shared memory region. A B
A: syscall to post a B: syscalls to receive an message to B (e.g., a incoming message. message queue). Wait Wait for request. for reply.
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 generated from an Interface Description Language (IDL)
Establishing an RPC connection to a named remote interface is often called binding. 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. 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 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
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.
[OpenGroup, late 1980s] Thread pool: idealized
worker Magic elastic worker pool loop Resize worker pool to match incoming request load: create/ handler Handle one destroy workers as needed. event, dispatch blocking as necessary.
Incoming handler idle workers request When handler (event) is complete, queue return to Workers wait here for next worker pool. request dispatch.
(Workers are threads.) handler Event/request queue
We can synchronize an event worker queue with a monitor: a loop mutex/CV pair. Protect the event queue data handler Handle one structure itself with the mutex. event, dispatch blocking as necessary.
Incoming event handler threads waiting on CV When handler queue is complete, return to Workers wait on the CV for worker pool. next event if the event queue is empty. Signal the CV when a new event arrives. This is a handler producer/consumer problem. 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
client host
NIC device
Internet “cloud” client kernel server hosts applica ons network with server so ware Data is sent on the network as messages applica ons
called packets. A simple, familiar example
request “GET /images/fish.gif HTTP/1.1”
reply
client (initiator) server
sd = socket(…); s = socket(…); connect(sd, name); bind(s, name); write(sd, request…); sd = accept(s); read(sd, reply…); read(sd, request…); close(sd); write(sd, reply…); close(sd); End-to-end data transfer
sender receiver
move data from move data from application to system buffer to system buffer application buffer queues buffer queues (mbufs, skbufs) TCP/IP protocol TCP/IP protocol
compute checksum compare checksum packet queues packet queues network driver network driver
DMA + interrupt DMA + interrupt transmit packet to deposit packet in network interface host memory Ports and packet demultiplexing
Data is sent on the network in messages called packets addressed to a des na on node and port. Kernel network stack demul plexes incoming network traffic: choose process/socket to receive it based on des na on port.
Apps with open sockets Incoming network packets
Network adapter hardware aka, network interface controller (“NIC”) Wakeup from interrupt handler
trap or fault return to user mode
sleep ready queue queue sleep wakeup switch
interrupt
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 a er N ms have elapsed.
Note: it isn’t actually the interrupt itself that wakes the thread, but the interrupt handler (so ware). 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
process user space syscall stub user buffers read() {…} trap Return syscall copyout copyin to user dispatch mode table
read() {…} I/O descriptor write() {…} table kernel 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
Several of the structural features used to improve RPC performance collapse layers of abstrac on. Programming a fast RPC is not for the squeamish. The slower path through the opera ng-system address space is used when the interrupt rou ne 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. Performance of Firefly RPC Michaels Schroeder and Burrows 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 expecta on that the 83% of the me not spent on the wire will decrease by a factor of 3. Looking at Table 3, however, we see that much of the RPC me goes to func ons that may not benefit propor onally from modern architectures. ……The only real ‘computa on” in RPC, in the tradi onal sense, is the checksum processing, and this in fact is memory-intensive and not compute- intensive; each checksum addi on is paired with a load …. Thus, Ousterhout found in the Sprite opera ng system [Ousterhout et al. 88] that kernel-to-kernel null RPC me was reduced by only half when moving from a Sun-3/75 to a SPARCsta on-l, even though integer performance increased by a factor of five [Ousterhout 90a].
Android: object-based RPC channels
Services register to Activity advertise for Manager JVM+lib clients. Service etc. Bindings are reference-counted. A client binds to a service. JVM+lib
Android binder an add-on kernel driver for /dev/binder object RPC Linux kernel Android services and libraries communicate by sending messages through shared-memory channels set up by binder. 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. …”
[http://developer.android.com/reference/android/os/IBinder.html] 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).