Skip to main content

Bottom half and techniques to implement it

Bottom half and techniques to implement

Bottom half" is a generic operating system term referring to the deferred portion of interrupt processing, so named because it represents the second, or bottom, half of the interrupt processing solution. In Linux, the term currently has this meaning, too. All the kernel's mechanisms for deferring work are "bottom halves." Some people also confusingly call all bottom halves "softirqs," but they are just being annoying. "Bottom half" also refers to the original deferred work mechanism in Linux. This mechanism is also known as a "BH," so we call it by that name now and leave the former as a generic description. The BH mechanism was deprecated a while back and fully removed in 2.5. Currently, there are three methods for deferring work: softirqs, tasklets, and work queues. Tasklets are built on softirqs and work queues are entirely different.

Using Softirqs
Softirqs are reserved for the most timing-critical and important bottom-half processing on the system. Currently only two subsystems networking and SCSIdirectly use softirqs. Additionally, kernel timers and tasklets are built on top of softirqs. If you are adding a new softirq, you normally want to ask yourself why using a tasklet is insufficient. Tasklets are dynamically created and are simpler to use because of their weaker locking requirements, and they still perform quite well. Nonetheless, for timing-critical applications that are able to do their own locking in an efficient way, softirqs might be the correct solution.

Tasklets
Tasklets are a bottom-half mechanism built on top of softirqs. As already mentioned, they have nothing to do with tasks. Tasklets are similar in nature and work in a similar manner to softirqs; however, they have a simpler interface and relaxed locking rules. The decision between whether to use softirqs versus tasklets is simple: You usually want to use tasklets. Softirqs are required only for very high-frequency and highly threaded uses. Tasklets, on the other hand, see much greater use. Tasklets work just fine for the vast majority of cases and they are very easy to use.

In most cases, tasklets are the preferred mechanism with which to implement your bottom half for a normal hardware device. Tasklets are dynamically created, easy to use, and very quick. Moreover, although their name is mind-numbingly confusing, it grows on you: It is cute.

Work Queues
Work queues are a different form of deferring work from what we have looked at so far. Work queues defer work into a kernel thread this bottom half always runs in process context. Thus, code deferred to a work queue has all the usual benefits of process context. Most importantly, work queues are schedulable and can therefore sleep.

Normally, it is easy to decide between using work queues and softirqs/tasklets. If the deferred work needs to sleep, work queues are used. If the deferred work need not sleep, softirqs or tasklets are used.

Comments

Popular posts from this blog

Dotnet, .Net 3.5, 2.0, C# Interview Questions

Few questions on dotnet, C# 2.0, 3.5 On Object oriented concepts 1)What is inheritance with e.g 2)What is polymorphism -function overloading -Function overriding -virtual keyword use -Static keyword and use -Abstract classes -Interface -Object 3)What is threading and how do we use in realtime application(cognizant) 4)What is threadpooling, lock, monitor(write code sample) 5)Architecture of current project 6)Session state, diffrent types of state management. 7)What is Application_Start, how it works. 8)Type of authentication in asp.net 9)How to configure ASP.NET application. 10) What is Impersonation. 11) What is WebService, WSDL, UDDI, Discovery, asmx files. 12) How to implement WebService and use it. 13) When to use WebServices. 14) WPF, how to implement(BOA) 15) Testing concvepts. 16) Test attributes 17) Flow of Automation Test Method execution 18) Features of dotnet 3.5 19) CLR, garbage collection 20) Finally block 21) Manifest, Metadata, MSIL 22) Assemblies, Type of assemblies, str...

Linux SMB write performance With Simple Tips

SMB write performance can be increased by Tuning the buffer cache. The secret to good performance is to keep as much of the data in memory for as long as is possible. Writing to the disk is the slowest part of any filesystem. If you know that the filesystem will be heavily used, then you can tune this process for Linux Samba. writing out dirty blocks to the disk until the filesystem buffer cache is 80 percent full (80). default is 40%, source = http://tldp.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/chap29sec287.html by writing echo 80 > /proc/sys/vm/dirty_ratio I am getting around 2MB increase while write operation, tested in Xp. I have tried with this single option, as the ref source is for linux 2.2 and we are using 2.6 kernel. we can try out Linux General Optimization suggested at http://tldp.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/gen-optim.html Tried with smb.conf, I am getting around 1MB gain while read and write. socket options = TCP_NODELAY I...

The Linux Foundation Free Training Program at linuxfoundation

The Linux Foundation Training Program is: * For the Community, by the Community. The Linux Foundation is building the program with its Technical Advisory Board to ensure the content, instructors and classes are the top quality available. * Technically the most advanced. Since the Linux Foundation works directly with community developers, it can cover features and advances in Linux before commercial companies. * Connected. The Linux Foundation has unfettered access to the leading developers and companies in the Linux ecosystem and will use these connections to best position attendees for success. For example, attendees can attend the exclusive, invite-only Collaboration Summit where they can forge connections beneficial to their career. * Real World. The Linux Foundation training courses all have hands on components and a highly rigorous curriculum of programming or administration exercises. Graduates will be well equipped to master Linux programming and system administr...