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Fri, 07 Dec 2007

The return of syslets.

Zach Brown announced new syslet patchset aimed to simplify and stbilize basic async operations. Syslets is a mechanims of performing syscalls asynchronously - new thread is started when syscall is about to block, execution blocks and old thread is scheduled away to the new one, on behalf of which userspace continues its execution.
Version 7 of the patchset was built on top of indirect syscall, threadlets, userspace function execution and async io was removed from the patchset for simplicity, number of comments and code clarifications were added.
Main goal of the syslets right now is to make fundamental things working right.

Asynchronous IO operations has too long history already - it was implemented as a state machine in KAIO and kevent AIO, kernel supports AIO for directIO operations (userspace requires libaio).
Syslet approach was shown to be in some cases much slower than libaio (which is actually a sync operations for usual files), but it was resolved as unfairness of CFS scheduler, and (iirc) it was fixed/extended.

My main objection against this is the fact, that when you have thousands of actively running applications, system starts sucking badly, but if it is possible to reduce maximum amount of working thread per user to some resonable limit, things will be just fine. Syslets (and its more friendly threadlets user) were supported by Linus and Ingo Molnar, so very likely it will be the default way to do asynchronous IO and other operations.
Right now Zach highlighted following problems:

• ring buffer of syslet statuses limitations
• ptrace() problems
• stale data (when thread issuing a syslet calls for example setuid(), in which case another thread, which actually executes blocked syscall, contains wrong data)
• problems with sys_clone() and syslets, sys_clone() is actually a mechanism to create a new thread in syslets, so we get a recursion

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