thread

## thread -- AOLserver Thread Library

    $Header: /cvsroot/aolserver/aolserver/thread/thread.html,v 1.1 2000/10/10 18:03:52 kriston Exp $

Theory of Operation
Known Issues
Sample Configuration

### Theory of Operation

The implementation of the underlying thread interface can have a noticeable effect on the overal performance of AOLserver. While AOLserver runs on many platforms, not all platforms perform the same with the same hardware, e.g., the same Intel hardware provide very diferent throughput and latency on Linux and FreeBSD.
A major influence on the overall performance is the scheduling scope of the thread library. Multithreading scope is often described as having a "1-1", "1-n", or "n-m" model. 1-1 means each thread is scheduled by the kernel along with all other threads. 1-n means one kernel thread is actually used by all process threads and thread context switching occurs in a user-level library. n-m means some number of kernel threads share the load of some larger number of process threads. The nature of the AOLserver workload (e.g., many simultaneous, I/O and system call bound threads) has shown that the 1-1 model generally works best. 1-n often does not provide enough concurrency and n-m introduces library overhead. In general the OS vendors go to great lengths to make sure the kernel scheduling algorithms perform very well under a variety of circumstance whereas library implementations are generally more simplistic in design, forced to share resources with many non-threaded processes, or require careful wrapping of many system calls with non-blocking I/O and signal handling. Heavily-loaded sites may realize better performance a change in configuration, e.g., upgrading to HP/11 from HP/10 or running SGI sproc-based nsd instead of the pthread-based nsd.
However, this does not mean that 1-n or n-m platforms should be avoided or you should not run AOLserver unless your platform is 1-1. In general, if you experience reasonable performance you do not need to worry about the performance of the underlying thread library. In fact, there are cases where the 1-n or n-m model may outperform, e.g., code which is somewhat compute bound and subject to lock contention. The reason is a 1-n or n-m platform can switch threads when it encounters a held lock and continue to make progress whereas a 1-1 thread must have the kernel put the thread on a lower level wait queue, generally a more expensive operation. Another example is a large-scale hosting environment where many customers share a single machine. You may find better scalability with 1-n threads than 1-1 on the same hardware as each low-traffic customer would require a single Unix process instead of six or more for base operation.
Listed below are the available and currently used threading models on each platform AOLserver runs on:

    Platform:	Available:	Used:	Notes:

    Solaris,	1-1, n-m	1-1	1-1 appears to perform better.
    UnixWare

    Linux		1-1		1-1	clone()-based LinuxThreads.

    SGI pthread	n-m		n-m	1-1 currently cannot be enabled.

    SGI sproc	1-1		1-1	sproc-based custom interface often
    					provides better performance then
    					pthread-interface.

    HP/10		1-n		1-n	Performs poorly under load.

    HP/11		1-1		1-1	n-m currently cannot be enabled.

    FreeBSD		1-n		1-n	rfork()-based 1-1 interface may be
    					available shortly.

    Apple OS/X	?		?	Likely 1-1.

    DEC Unix 4.0	1-1, n-m	n-m	1-1 can be enabled but blows up
    					after many thread creates/exits.
    					n-m model performs well.

    Windows NT	1-1		1-1	New Fiber() Win32 API may provide
    					n-m in the future.

### Known Issues

We really want to make libnsthread.a into libnsthread.so and take it out of nsd. This has already happened for Win32, so it should be a relatively simple matter for Unix.

### Sample Configuration

The library itself has no "configuration" but AOLserver has configuration parameters that affect the thread library. Please refer to the Configuration File Referencefor extensive threading parameters for AOLserver.