Re: [ANNOUNCE] Native POSIX Thread Library 0.1

Bill Huey
2002-09-23T22:56:25Z

On Mon, Sep 23, 2002 at 06:44:23PM -0400, Mark Mielke wrote:
> Think of it this way... two threads are blocked on different resources...
> The currently executing thread reaches a point where it blocks.
>
> OS threads:
> 1) thread#1 invokes a system call
> 2) OS switches tasks to thread#2 and returns from blocking
>
> user-space threads:
> 1) thread#1 invokes a system call
> 2) thread#1 returns from system call, EWOULDBLOCK

> 3) thread#1 invokes poll(), select(), ioctl() to determine state
> 4) thread#1 returns from system call

More like the UTS blocks the thread and waits for an IO upcall to notify
the change of state in the kernel. It's equivalent to a single in overhead,
something like a SIGIO, or async IO notification.

Delete 3 and 4. It's certainly much faster than select() and family.

> 5) thread#1 switches stack pointer to be thread#2 upon determination
> that the resource thread#2 was waiting on is ready.

Right, then marks it running and runs it.

> Certainly the above descriptions are not fully accurate, or complete,
> and it is possible that the M:N threading would make a fair compromise
> between OS thread sand user-space threads, however, if user-space threads
> requires all this extra work, and M:N threads requires some extra work,
> some less work, and extra book keeping and system calls, why couldn't
> OS threads by themselves be more efficient?

Crazy synchronization by non-web-server like applications. Who knows. I
personally can't think up really clear example at this time since I don't
do that kind of programming, but I'm sure concurrency experts can...

I'm just not one of those people.

bill