Home > Articles > Operating Systems, Server > Solaris

  • Print
  • + Share This
Like this article? We recommend

Small Packet TCP Traffic Performance

This section studies the gigabit performance of TCP with small packet network traffic. The ce interface cards are used for the evaluation. The same set of hardware and software configurations that were used for "Bulk Transfer Traffic Performance" is the test bed. Note that a request/response type of traffic is not investigated, but rather a continuous unidirectional flow of small packets.

Effect of Nagle's Algorithm and Deferred Acknowledgment

As discussed in "Small Packet Traffic Issues," Nagle's algorithm plays an important role in the transmission of small packets. Since Nagle's algorithm asks the sender to accumulate packets when there is one unacknowledged small packet outstanding, packets sent from the application may not be put on the wire as soon as they arrive in TCP. In the meantime, systems at the receiving end typically enable deferred acknowledgment in the hope of having optimal throughput in the bulk transfer case. Hence, if an application is trying to send a series of small messages (less than 1,460 bytes), these messages may not be delivered immediately. Instead, these messages may be delivered with visible delays.

Table 2 Performance Of Small Packet Traffic When The Sender Turns On Nagle's Algorithm and the Receiver Enables Deferred Acknowledgment

Message (bytes)

On-wire packet payload (bytes)

Packet rate

Throughput (Mbps)

60

60, 1440

13198

79.20

100

100, 1400

15224

91.35

250

250, 1250

26427

181.56

536

536,1072

36548

235.09

1024

1024

42417

347.49

1460

1460

57146

667.48


This section explains what may be happening, but first looks at the raw packet rate the ce interface is capable of delivering when the sender adopts Nagle's algorithm and the receiver activates deferred acknowledgment. Note that the receiver sets tcp_deferred_acks_max to eight in this case. TABLE 2 lists the packet rate and throughput for a ce card when the server only sends packets. The throughput and packet rate goes up as the message size increases. However, the packet rate of a 60-byte payload is only about one-fifth of the packet rate of a 1,460-byte payload. To understand what causes the low packet rate for small packets, the snoop utility in the Solaris 8 OE was used. FIGURE 8 shows what was found in the 100-byte payload case. After the network ramps up, that is, beyond the slow start [4] phase, a cycle like this is seen. First, the server (Machine S) sends the client (Machine C) a packet with 100 bytes of payload. Machine S cannot continue sending without waiting for an ACK packet from machine C because the last packet it sent had less than 1,460 bytes of payload. Machine C, however, is waiting for more packets from S to reduce the number of ACK packets per data packet. In the mean time, machine S accumulates data from the application. Finally, the amount of unsent data in machine S reaches 1500 bytes (15 packets of 100 bytes), that is, above the 1,460-byte MSS, and sends out a packet with 1400 bytes of payload. Note that machine S will not fragment packets to have a packet with a 1,460-byte payload. When machine C gets the packet with 1400-byte payload, it immediately sends machine S an ACK packet, then this cycle restarts.

Figure 8FIGURE 8 Sending Process Of Packets With 100-byte Payload When Nagle's Algorithm is Enabled at the Sender (S) and Deferred Acknowledgment is Enabled at the Receiver (C)

Now one question pops up. Why doesn't C wait until it receives an amount of data no smaller than eight MSS before it sends out an ACK? Obviously, this could have made the interlocking scenario worse, since machine S would have to accumulate another 1500 bytes before it could send out a packet with 1400 bytes of payload. Solaris 8 OE actually takes care of this situation gracefully by enforcing the following rules. In summary, the receiver will send out an ACK packet immediately if both of the following conditions are true:

  • A non-MSS segment arrived.

  • The amount of unacknowledged data is not a multiple of MSS.

Note that Solaris OE is trying to address the end-of-connection issue here, but it also affects the experiment.

After this scenario is explained, you can see that the relationship of the packet rate and throughput is as follows:

Throughput = Packet_Rate * (floor(1460/message_size) + 1) * message_size/2

where the floor function trims the decimal part of its parameter. The packet rate is mostly determined by the system's capability to execute system calls for moving data from user address space to the kernel address space. Obviously, to obtain better performance for small packets, the system must disable Nagle's algorithm or disable deferred acknowledgment. However, disabling deferred acknowledgment may negatively affect the performance of bulk transfer. Also, the opportunity to piggy-back acknowledgments with data packets from machine C to machine S, if machine C has any, may be lost. Hence, the preferred approach is to disable Nagle's algorithm only in the sender. The two ways to achieve this goal are:

  • Set tcp_naglim_def to one using the ndd command. TCP sends the packet downstream immediately when it receives a message from the application. If the communication involves only Sun servers and workstations, an ACK packet will be delivered after the server transmits two packets.

  • In the application, use the TCP_NODELAY option in the socket function call to create a socket. Only the application can know whether it will be communicating using small packets. Hence, it makes sense to ask the applications that use small packets to disable Nagle's algorithm for the particular sockets that they need. Disabling Nagle's algorithm in a system-wide manner is not preferred.

TABLE 3 shows the new packet rate for the same list of message sizes in TABLE 2 after Nagle's algorithm is disabled. The value of tcp_maxpsz_multiplier is set to 10 to produce the numbers in this table. The new packet rates for payloads of 60 bytes to 1,024 bytes increase by up to three times to a level close to the packet rate of the 1,460-byte payload (packets with Ethernet MTU size). Note that even though the packet rates are higher, the actual throughput is lower than the numbers shown in TABLE 2 because each packet now only carries the same amount of payload as the message size. However, no visible pauses will be observed during the transmission. The throughput and packet rate do not change much whether deferred acknowledgments are enabled or not. Since disabling deferred acknowledgment means higher overhead per data packet and the loss of opportunity to piggyback acknowledgment, disabling this feature is not recommended.

Table 3 Performance Of Small Packet Traffic When the Sender Turns Off Nagle's Algorithm

Payload (bytes)

Deferred-ack on packet rate

Throughput (Mbps)

Deferred-ack off packet rate

Throughput(Mbps)

60

37224

25.90

39010

26.36

100

41987

38.15

43736

41.15

250

41297

102.87

42193

99.76

536

43724

188.66

41861

180.15

1024

42576

348.78

41024

336.07

1460

57527

671.92

57554

672.24


Packet Rate Versus Message Size

Traditionally, the packet processing cost is divided into two parts, the cost associated with processing a packet of minimal size, and the cost associated with moving data from the kernel buffer to the user buffer [7]. The former is called per-packet cost and the latter is called per-byte cost. Under this model, larger packets always take longer to process. This model was developed when the bandwidth of the system backplane was low. However, the current Sun™ Fireplane interconnect can support 9.6 gigabytes per second sustained throughput. This may make the per-byte cost trivial. As a result, the per-packet cost can dominate in processing each packet, which makes the processing time for packets of different sizes very close.

To see the new relationship between packet size and packet rate, the following experiment was conducted:

  • Measure packet rates with payload ranging from one byte to 1,460 bytes.

  • Tune the system so that one system call from the user application to send a packet corresponds to one packet on wire. This is done by disabling the Nagle's algorithm (setting tcp_naglim_def to 1).

  • Tune the system so that only one packet is moved between kernel components and between kernel and user applications. This is done by setting tcp_maxpsz_multiplier to 1 on the sending side (server) and setting tcp_deferred_acks_max to 1 on the receiving side (client).

  • The server only transmits and only one CPU is enabled.

FIGURE 9 shows how the packet rates change when the payload varies from one byte to 1,460 bytes. The numbers shown in this figure are expected to be lower than those shown in TABLE 2 due to the special setting used previously. The packet rates for message sizes of 180 bytes and smaller stay very close for the ce card. The packet rates for message sizes of 250 bytes and beyond are very close also. However, the packet rate for message size of 1,460 bytes (the packets on wire will be full-size Ethernet frames) is only 25 percent lower than those of 180-byte or smaller messages. TABLE 3 shows the percentage of CPU time for user- mode and user-to kernel copy in some of the preceding test cases. Not surprisingly, copy cost is below 10 percent across the board.7 These findings indicate that the cost associated with copying data in the operating system (which increases more than 350 times from four bytes to 1,460 bytes) is not dominant on Sun Fire 6800 platform. It is the cost associated to process each packet that affects performance most.

Table 4 Percentage of CPU Time for User-Mode and Kernel-to-User Copy

Payload (bytes)

Percent user time

Percent kernel to user copy

100

9

1

180

9

2

250

9

3

536

8

5

1024

6

5

1460

6

7


Figure 9FIGURE 9 Percentage of CPU Time for User-Mode and Kernel-to-User Copy

  • + 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