The LWKT scheduler is responsible for actually running a thread. It uses a fixed priority scheme, but the fixed priorities are differentiating major subsystems, not user processes. For example, hardware interrupt threads have the highest priority, followed by software interrupts, kernel-only threads, then finally user threads. A user thread either runs at user-kernel priority (when it is actually running in the kernel, e.g. running a syscall on behalf of userland), or a user thread runs at user priority.
DragonFly does preempt, it just does it very carefully and only under particular circumstances. An LWKT interrupt thread can preempt most other threads, for example. This mimics what FreeBSD-4.x already did with its spl/run-interrupt-in-context-of-current-process mechanism. What DragonFly does *NOT* do is allow a non-interrupt kernel thread to preempt another non-interrupt kernel thread.
The mainframe z/OS Operating system supports a similar mechanism, called SRB (Service Request Block).
SRB's represent requests to execute a system service routine. SRB's are typically created when one address space detects an event that affects a different address space; they provide one of several mechanisms for asynchronous inter-address space communication for programs running on z/OS.
An SRB is similar to a Process Control Block (PCB), in that it identifies a unit of work to the system. Unlike a PCB, an SRB cannot "own" storage areas. In a multiprocessor environment, the SRB routine, after being scheduled, can be dispatched on another processor and can run concurrently with the scheduling program. The scheduling program can continue to do other processing in parallel with the SRB routine. Only programs running in kernel mode can create an SRB.
The Windows Operating System knows a similar light weight thread mechanism named "fibers". Fibers are scheduled by an application program. The port of the CICS Transaction Server to the Windows platform uses fibers, somewhat analogous to the use of "enclaves" under z/OS.
In UNIX, "kernel threads" have two threads, one is the core thread, one is the user thread.
light, weight, kernel, threads, this, article, technical, most, readers, understand, please, help, improve, make, understandable, experts, without, removing, technical, details, august, 2014, learn, when, remove, this, message, lwkt, computer, science, term, f. This article may be too technical for most readers to understand Please help improve it to make it understandable to non experts without removing the technical details August 2014 Learn how and when to remove this message Light Weight Kernel Threads LWKT is a computer science term and from DragonFly BSD in particular LWKTs differ from normal kernel threads in that they can preempt normal kernel threads According to Matt Dillon DragonFlyBSD creator The LWKT scheduler is responsible for actually running a thread It uses a fixed priority scheme but the fixed priorities are differentiating major subsystems not user processes For example hardware interrupt threads have the highest priority followed by software interrupts kernel only threads then finally user threads A user thread either runs at user kernel priority when it is actually running in the kernel e g running a syscall on behalf of userland or a user thread runs at user priority DragonFly does preempt it just does it very carefully and only under particular circumstances An LWKT interrupt thread can preempt most other threads for example This mimics what FreeBSD 4 x already did with its spl run interrupt in context of current process mechanism What DragonFly does NOT do is allow a non interrupt kernel thread to preempt another non interrupt kernel thread The mainframe z OS Operating system supports a similar mechanism called SRB Service Request Block SRB s represent requests to execute a system service routine SRB s are typically created when one address space detects an event that affects a different address space they provide one of several mechanisms for asynchronous inter address space communication for programs running on z OS An SRB is similar to a Process Control Block PCB in that it identifies a unit of work to the system Unlike a PCB an SRB cannot own storage areas In a multiprocessor environment the SRB routine after being scheduled can be dispatched on another processor and can run concurrently with the scheduling program The scheduling program can continue to do other processing in parallel with the SRB routine Only programs running in kernel mode can create an SRB The Windows Operating System knows a similar light weight thread mechanism named fibers Fibers are scheduled by an application program The port of the CICS Transaction Server to the Windows platform uses fibers somewhat analogous to the use of enclaves under z OS In UNIX kernel threads have two threads one is the core thread one is the user thread See also editLight weight process Thread computing Sources editMatt Dillon s post about the LWKT scheduler Kerner Sean Michael 2006 01 10 New DragonFly Released For BSD Users InternetNews retrieved 2011 11 20 Biancuzzi Federico 2004 07 08 Behind DragonFly BSD O Reilly Media archived from the original on 2014 04 09 retrieved 2011 11 20 Loli Queru Eugenia 2004 03 13 Interview with Matthew Dillon of DragonFly BSD OSNews Luciani Robert 2009 05 24 M N threading in DragonflyBSD PDF BSDCon archived from the original PDF on 2010 12 23 Andrews Jeremy 2007 08 06 Interview Matthew Dillon KernelTrap archived from the original on 2011 05 15 retrieved 2011 11 20 Vervloesem Koen 2010 04 21 DragonFly BSD 2 6 towards a free clustering operating system LWN net retrieved 2011 11 19 Kerner Sean Michael 2006 07 25 DragonFly BSD 1 6 Cuts the Cord InternetNews retrieved 2011 11 20 Townsend Trent 2006 01 18 A Quick Review of DragonFly BSD 1 4 OSNews retrieved 2011 11 16 Interju Matthew Dillionnal a DragonFly BSD alapitojaval 2003 10 10 retrieved 2011 11 20 nbsp This operating system related article is a stub You can help Wikipedia by expanding it vte Retrieved from https en wikipedia org w index php title Light Weight Kernel Threads amp oldid 1146629036, wikipedia, wiki, book, books, library,
