Home > Articles > Web Services > Cloud Computing

  • Print
  • + Share This
This chapter is from the book

1.7 Speed

So far we have elaborated on many of the considerations involved in designing large distributed systems. For web and other interactive services, one item may be the most important: speed. It takes time to get information, store information, compute and transform information, and transmit information. Nothing happens instantly.

An interactive system requires fast response times. Users tend to perceive anything faster than 200 ms to be instant. They also prefer fast over slow. Studies have documented sharp drops in revenue when delays as little as 50 ms were artificially added to web sites. Time is also important in batch and non-interactive systems where the total throughput must meet or exceed the incoming flow of work.

The general strategy for designing a system that is performant is to design a system using our best estimates of how quickly it will be able to process a request and then to build prototypes to test our assumptions. If we are wrong, we go back to step one; at least the next iteration will be informed by what we have learned. As we build the system, we are able to remeasure and adjust the design if we discover our estimates and prototypes have not guided us as well as we had hoped.

At the start of the design process we often create many designs, estimate how fast each will be, and eliminate the ones that are not fast enough. We do not automatically select the fastest design. The fastest design may be considerably more expensive than one that is sufficient.

How do we determine if a design is worth pursuing? Building a prototype is very time consuming. Much can be deduced with some simple estimating exercises. Pick a few common transactions and break them down into smaller steps, and then estimate how long each step will take.

Two of the biggest consumers of time are disk access and network delays.

Disk accesses are slow because they involve mechanical operations. To read a block of data from a disk requires the read arm to move to the right track; the platter must then spin until the desired block is under the read head. This process typically takes 10 ms. Compare this to reading the same amount of information from RAM, which takes 0.002 ms, which is 5,000 times faster. The arm and platters (known as a spindle) can process only one request at a time. However, once the head is on the right track, it can read many sequential blocks. Therefore reading two blocks is often nearly as fast as reading one block if the two blocks are adjacent. Solid-state drives (SSDs) do not have mechanical spinning platters and are much faster, though more expensive.

Network access is slow because it is limited by the speed of light. It takes approximately 75 ms for a packet to get from California to the Netherlands. About half of that journey time is due to the speed of light. Additional delays may be attributable to processing time on each router, the electronics that convert from wired to fiber-optic communication and back, the time it takes to assemble and disassemble the packet on each end, and so on.

Two computers on the same network segment might seem as if they communicate instantly, but that is not really the case. Here the time scale is so small that other delays have a bigger factor. For example, when transmitting data over a local network, the first byte arrives quickly but the program receiving the data usually does not process it until the entire packet is received.

In many systems computation takes little time compared to the delays from network and disk operation. As a result you can often estimate how long a transaction will take if you simply know the distance from the user to the datacenter and the number of disk seeks required. Your estimate will often be good enough to throw away obviously bad designs.

To illustrate this, imagine you are building an email system that needs to be able to retrieve a message from the message storage system and display it within 300 ms. We will use the time approximations listed in Figure 1.10 to help us engineer the solution.

Figure 1.10

Figure 1.10: Numbers every engineer should know

First we follow the transaction from beginning to end. The request comes from a web browser that may be on another continent. The request must be authenticated, the database index is consulted to determine where to get the message text, the message text is retrieved, and finally the response is formatted and transmitted back to the user.

Now let’s budget for the items we can’t control. To send a packet between California and Europe typically takes 75 ms, and until physics lets us change the speed of light that won’t change. Our 300 ms budget is reduced by 150 ms since we have to account for not only the time it takes for the request to be transmitted but also the reply. That’s half our budget consumed by something we don’t control.

We talk with the team that operates our authentication system and they recommend budgeting 3 ms for authentication.

Formatting the data takes very little time—less than the slop in our other estimates—so we can ignore it.

This leaves 147 ms for the message to be retrieved from storage. If a typical index lookup requires 3 disk seeks (10 ms each) and reads about 1 megabyte of information (30 ms), that is 60 ms. Reading the message itself might require 4 disk seeks and reading about 2 megabytes of information (100 ms). The total is 160 ms, which is more than our 147 ms remaining budget.

While disappointed that our design did not meet the design parameters, we are happy that disaster has been averted. Better to know now than to find out when it is too late.

It seems like 60 ms for an index lookup is a long time. We could improve that considerably. What if the index was held in RAM? Is this possible? Some quick calculations estimate that the lookup tree would have to be 3 levels deep to fan out to enough machines to span this much data. To go up and down the tree is 5 packets, or about 2.5 ms if they are all within the same datacenter. The new total (150 ms+3 ms+2.5 ms+100 ms = 255.5 ms) is less than our total 300 ms budget.

We would repeat this process for other requests that are time sensitive. For example, we send email messages less frequently than we read them, so the time to send an email message may not be considered time critical. In contrast, deleting a message happens almost as often reading messages. We might repeat this calculation for a few deletion methods to compare their efficiency.

One design might contact the server and delete the message from the storage system and the index. Another design might have the storage system simply mark the message as deleted in the index. This would be considerably faster but would require a new element that would reap messages marked for deletion and occasionally compact the index, removing any items marked as deleted.

Even faster response time can be achieved with an asynchronous design. That means the client sends requests to the server and quickly returns control to the user without waiting for the request to complete. The user perceives this system as faster even though the actual work is lagging. Asynchronous designs are more complex to implement. The server might queue the request rather than actually performing the action. Another process reads requests from the queue and performs them in the background. Alternatively, the client could simply send the request and check for the reply later, or allocate a thread or subprocess to wait for the reply.

All of these designs are viable but each offers different speed and complexity of implementation. With speed and cost estimates, backed by prototypes, the business decision of which to implement can be made.

  • + Share This
  • 🔖 Save To Your Account

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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