Home > Articles

This chapter is from the book

Item 30: Use Async Methods to Avoid Thread Allocations and Context Switches

It’s all too easy to begin thinking that all asynchronous tasks represent work being done on other threads. After all, that is one use for asynchronous work. But, in fact, many times asynchronous work doesn’t start a new thread. File I/O is asynchronous, but uses I/O completion ports rather than threads. Web requests are asynchronous, but use network interrupts rather than threads. In these instances, using async tasks frees a thread to do useful work.

When you offload work to another thread, you free up one thread at the cost of creating and running another. That’s a wise design only when the thread you are freeing up is a scarce resource. In a GUI application, the UI thread is a scarce resource: Only one thread interacts with any of the visual elements the user sees. However, thread pool threads are neither unique nor scarce (although they are limited in number); that is, one thread is the same as any other thread in the same pool. For that reason, you should avoid CPU-bound async tasks in non-GUI applications.

To explore this issue further, let’s start with GUI applications. When the user initiates an action from the UI, she expects the UI to remain responsive. It won’t be if the UI thread spends seconds (or more) performing the last action. The solution to this problem is to offload that work to another resource so that the UI can remain responsive to other user actions. As you saw in Item 29, these UI event handlers are one of the locations where async over sync composition makes sense.

Now let’s move on to console applications. Console applications that perform only one long-running CPU-bound task will not benefit from executing that work on a separate thread. The main thread will be synchronously waiting, and the worker thread will be busy. In such a case, you tie up two threads to do the work of one.

However, if a console application performs several long-running CPU-bound operations, it may make sense to run those tasks on separate threads. Item 35 discusses several options to run CPU-bound work on multiple threads.

That brings us to ASP.NET Server applications, which seem to generate a great deal of confusion among developers. Ideally, you want to keep threads free so that your application can handle a greater number of incoming requests. That leads to a design where you would offload the CPU-bound work to different threads in your ASP.NET handlers:

public async Task<IActionResult> Compose()
var model = await LongRunningCPUTask();
return View(model);

Let’s examine the details of what happens in this situation. By starting another thread for the task, you allocate a second thread from the thread pool. The first thread has nothing to do and can be recycled and given more work, but that requires more overhead. To “bring you back where you were,” the SynchronizationContext must keep track of all the state for this Web request and, when the awaited CPU-bound work completes, restore that state. Only then can the handler respond to the client.

With this approach, you haven’t freed any resources, but you’ve added two context switches when processing a request.

If you have long-running CPU-bound work to do in response to Web requests, you need to offload that work to another process or another machine so as to free up the thread resource and increase your Web application’s ability to service requests. For example, you might have a second Web job that receives CPU-bound requests and executes them in turn. Alternatively, you might allocate a second machine to the CPU-bound work.

Which option is the fastest depends on the characteristics of your application: the amount of traffic, the time needed to do the CPU-bound work, and network latency. You must measure these items to make an informed decision. One of the configurations you should measure is doing all the work in the Web application synchronously. This approach will likely be faster than offloading the work to another thread in the same thread pool and process.

Asynchronous work seems like magic: You offload work to another location, and then pick up your processing after it completes. To ensure the efficiency of this approach, you need to make sure that when you offload work, you free up resources rather than simply switch contexts between similar resources.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020