Performance Tuning

Apache Traffic Server™ in its default configuration should perform suitably for running the included regression test suite, but will need special attention to both its own configuration and the environment in which it runs to perform optimally for production usage.

There are numerous options and strategies for tuning the performance of Traffic Server and we attempt to document as many of them as possible in the sections below. Because Traffic Server offers enough flexibility to be useful for many caching and proxying scenarios, which tuning strategies will be most effective for any given use case may differ, as well as the specific values for various configuration options.

Before You Start

One of the most important aspects of any attempt to optimize the performance of a Traffic Server installation is the ability to measure that installation’s performance; both prior to and after any changes are made. To that end, it is strongly recommended that you establish some means to monitor and record a variety of performance metrics: request and response speed, latency, and throughput; memory and CPU utilization; and storage I/O operations.

Attempts to tune a system without being able to compare the impact of changes made will at best result in haphazard, feel good results that may end up having no real world impact on your customers’ experiences, and at worst may even result in lower performance than before you started. Additionally, in the all too common situation of budget constraints, having proper measurements of existing performance will greatly ease the process of focusing on those individual components that, should they require hardware expenditures or larger investments of employee time, have the highest potential gains relative to their cost.

Building Traffic Server

While the default compilation settings for Traffic Server will produce a set of binaries capable of serving most caching and proxying needs, there are some build options worth considering in specific environments.

Hardware Tuning

As with any other server software, efficient allocation of hardware resources will have a significant impact on Traffic Server performance.

CPU Selection

Apache Traffic Server™ uses a hybrid event-driven engine and multi-threaded processing model for handling incoming requests. As such, it is highly scalable and makes efficient use of modern, multicore processor architectures.

Memory Allocation

Though Traffic Server stores cached content within an on-disk host database, the entire Cache Directory is always maintained in memory during server operation. Additionally, most operating systems will maintain disk caches within system memory. It is also possible, and commonly advisable, to maintain an in-memory cache of frequently accessed content.

The memory footprint of the Traffic Server process is largely fixed at the time of server startup. Your Traffic Server systems will need at least enough memory to satisfy basic operating system requirements, as well as capacity for the cache directory, and any memory cache you wish to use. The default settings allocate roughly 10 megabytes of RAM cache for every gigabyte of disk cache storage, though this setting can be adjusted manually in records.config using the setting proxy.config.cache.ram_cache.size. Traffic Server will, under the default configuration, adjust this automatically if your system does not have enough physical memory to accomodate the aforementioned target.

Aside from the cost of physical memory, and necessary supporting hardware to make use of large amounts of RAM, there is little downside to increasing the memory allocation of your cache servers. You will see, however, no benefit from sizing your memory allocation larger than the sum of your content (and index overhead).

Disk Storage

Except in cases where your entire cache may fit into system memory, your cache nodes will eventually need to interact with their disks. While a more detailed discussion of storage stratification is covered in Cache Partitioning below, very briefly you may be able to realize gains in performance by separating more frequently accessed content onto faster disks (PCIe SSDs, for instance) while maintaining the bulk of your on-disk cache objects, which may not receive the same high volume of requests, on lower-cost mechanical drives.

Operating System Tuning

Apache Traffic Server™ is supported on a variety of operating systems, and as a result the tuning strategies available at the OS level will vary depending upon your chosen platform.

General Recommendations

TCP Keep Alive

TCP Congestion Control Settings

Ephemeral and Reserved Ports

Jumbo Frames



OmniOS / illumos

Mac OS X

Traffic Server Tuning

Traffic Server itself, of course, has many options you may want to consider adjusting to achieve optimal performance in your environment. Many of these settings are recorded in records.config and may be adjusted with the traffic_ctl config set command line utility while the server is operating.

CPU and Thread Optimization

Thread Scaling

By default, Traffic Server creates 1.5 threads per CPU core on the host system. This may be adjusted with the following settings in records.config:

Thread Affinity

On multi-socket servers, such as Intel architectures with NUMA, you can adjust the thread affinity configuration to take advantage of cache pipelines and faster memory access, as well as preventing possibly costly thread migrations across sockets. This is adjusted with proxy.config.exec_thread.affinity in records.config.

CONFIG proxy.config.exec_thread.affinity INT 1

Timeout Settings

Traffic Server has a variety of timeout settings which may be modified to help tune the performance of various proxy components. In general it is recommended to leave the timeouts at their default values unless you have identified specific causes for an adjustment.

Note that not all proxy configurations will be impacted by every timeout. For instance, if you are not using any hierarchical caching then the parent proxy timeouts will be irrelevant.

While all of the timeouts described below may be set globally for your Traffic Server instance using records.config, many of them are also overridable on a per-transaction basis by plugins (including Configuration Remap Plugin). This allows the possibility for adjusting timeout value for individual subsets of your cache.

For example, you may wish to be fairly lenient on activity timeouts for most of your cache, leaving the default at a minute or two, but enforce a much stricter timeout on a set of very small, incredibly heavily accessed objects for which you can construct a map rule with the goal of reducing the chances that a few bad actors (misconfigured or misbehaving clients) may generate too much connection pressure on your cache. The tradeoff may be that some perfectly innocent, but slow clients may have their connections terminated early. As with all performance tuning efforts, your needs are likely to vary from others’ and should be carefully considered and closely monitored.

Default Inactivity Timeout

The setting is applied to the HTTP state machine when no other inactivity timeouts have been applied. In effect, it sets an upper limit, in seconds, on state machine inactivity.

In addition to the timeout itself, there is a related statistic: which tracks the number of times the default inactivity timeout was applied to transactions (as opposed to a more specific timeout having been applied).


Accept Timeout

The variable proxy.config.http.accept_no_activity_timeout sets, in seconds, the time after which Traffic Server will close incoming connections which remain inactive (have not sent data). Lowering this timeout can ease pressure on the proxy if misconfigured or misbehaving clients are opening a large number of connections without submitting requests.

CONFIG proxy.config.http.accept_no_activity_timeout INT 120

Background Fill Timeout

When background fills are enabled, proxy.config.http.background_fill_active_timeout sets in seconds the time after which Traffic Server will abort the fill attempt and close the origin server connection that was being used. Setting this to zero disables the timeout, but modifying the value and enforcing a timeout may help in situtations where your origin servers stall connections without closing.

CONFIG proxy.config.http.background_fill_active_timeout INT 0

DNS Timeouts

Traffic Server performs all DNS queries for origin servers through the HostDB subsystem. Two settings affect the potential frequency and amount of time Traffic Server will spend on these lookups. proxy.config.hostdb.timeout is used to establish the time-to-live, in minutes, for all DNS records and proxy.config.hostdb.lookup_timeout sets, in seconds, the timeout for actual DNS queries.

Setting a higher timeout value will reduce the number of times Traffic Server needs to perform DNS queries for origin servers, but may also prevent your Traffic Server instance from updating its records to reflect external DNS record changes in a timely manner (refer to proxy.config.hostdb.ttl_mode for more information on when this TTL value will actually be used).

CONFIG proxy.config.hostdb.timeout INT 1440
CONFIG proxy.config.hostdb.lookup_timeout INT 30

Keepalive Timeouts

Traffic Server keepalive timeouts may be set both for maintaining a client connection for subsequent requests, using proxy.config.http.keep_alive_no_activity_timeout_in, as well as origin server connections for subsequent object requests (when not servable from the cache) using proxy.config.http.keep_alive_no_activity_timeout_out. Both are specified in seconds. Keep in mind that keep_alive_no_activity_timeout_out for origin server connections is effectively an advisory maximum, as the origin server may have its own keepalive timeout which (if set lower) will likely take precedence.

CONFIG proxy.config.http.keep_alive_no_activity_timeout_in INT 120
CONFIG proxy.config.http.keep_alive_no_activity_timeout_out INT 120

Origin Connection Timeouts

Origin server connection timeouts are configured with proxy.config.http.connect_attempts_timeout, which is applied both to the initial connection as well as any retries attempted, should an attempt timeout. The timeout applies from the moment Traffic Server begins the connection attempt until the origin returns the first byte.

In the case where you wish to have a different (generally longer) timeout for POST and PUT connections to an origin server, you may also adjust proxy.config.http.post_connect_attempts_timeout which applies only to origin connections using those HTTP verbs.

CONFIG proxy.config.http.connect_attempts_timeout INT 30
CONFIG proxy.config.http.post_connect_attempts_timeout INT 1800

Parent Proxy Timeout

In hierarchical caching configurations, the proxy.config.http.parent_proxy.connect_attempts_timeout setting is used for all connection attempts to parent caches. It may be useful, in cases where you wish to have Traffic Server fall back to an alternate parent cache (in configurations where you have multiple parents for the same cache) more quickly, to lower this timeout.

CONFIG  proxy.config.http.parent_proxy.connect_attempts_timeout INT 30

Polling Timeout

If you are experiencing unusually or unacceptably high CPU utilization during idle workloads, you may consider adjusting the polling timeout with


SOCKS Timeouts

In Traffic Server configurations where SOCKS has been enabled, three timeouts are made available for tuning. Basic activity timeout for SOCKS server connections may be adjusted with proxy.config.socks.socks_timeout, in seconds. Server connection attempts (initial connections attempts only) are covered by proxy.config.socks.server_connect_timeout, again in seconds, and server connection retry attempts are set with proxy.config.socks.server_retry_timeout. Note that the retry timeout is the timeout for the actual connection attempt on a retry, not the delay after which a retry will be performed (the delay is configured with proxy.config.socks.server_retry_time).

CONFIG proxy.config.socks.socks_timeout INT 100
CONFIG proxy.config.socks.server_connect_timeout INT 10
CONFIG proxy.config.socks.server_retry_timeout INT 300
CONFIG proxy.config.socks.server_retry_time INT 300

SSL Timeouts

Traffic Server offers a few timeouts specific to encrypted connections handled by the SSL engine.

proxy.config.ssl.handshake_timeout_in configures the time, in seconds, after which incoming client connections will abort should the SSL handshake not be completed. The default of 0 disables the timeout.

When OCSP Stapling is enabled in Traffic Server, you can configure two separate timeouts; one for setting the length of time which cached OCSP results will persist, specified in seconds using proxy.config.ssl.ocsp.cache_timeout, and the timeout for requests to the remote OCSP responders, in seconds, with proxy.config.ssl.ocsp.request_timeout.

Lastly, you can control the number of seconds for which SSL sessions will be cached in Traffic Server using proxy.config.ssl.session_cache.timeout.

CONFIG proxy.config.ssl.handshake_timeout_in INT 0
CONFIG proxy.config.ssl.ocsp.cache_timeout INT 3600
CONFIG proxy.config.ssl.ocsp.request_timeout INT 10
CONFIG proxy.config.ssl.session_cache.timeout INT 0

Transaction Activity Timeouts

Traffic Server specifies two sets of general transaction activity timeouts: a pair for active transactions, and a pair for inactive connections (ones which are not receiving or sending data during the timeout period). Each pair includes one timeout for client connections (the _in variant) and another for origin server transactions (_out variants).

For active transactions, proxy.config.http.transaction_active_timeout_in and proxy.config.http.transaction_active_timeout_out set the maximum time, in seconds, which Traffic Server will spend sending/receiving data with a client or origin server, respectively. If the data transfer has not completed within the time specified then the connection will be closed automatically. This may result in the lack of a cache update, or partial data transmitted to a client. Both timeouts are disabled (set to 0) by default.

In general, it’s unlikely you will want to enable either of these timeouts globally, especially if your cache contains objects of varying sizes and deals with clients which may support a range of speeds (and therefore take less or more time to complete normal, healthy data exchanges). However, there may be configurations in which small objects need to be exchanged in very short periods and you wish your Traffic Server cache to enforce these time resrictions by closing connections which exceed them.

The variables proxy.config.http.transaction_no_activity_timeout_in and proxy.config.http.transaction_no_activity_timeout_out control the maximum amount of time which Traffic Server will spend in a transaction which is stalled and not transmitting data, for clients and origin servers respectively.

Unlike the active transaction timeouts, these two inactive transaction timeout values prove somewhat more generally applicable.

CONFIG proxy.config.http.transaction_active_timeout_in INT 0
CONFIG proxy.config.http.transaction_active_timeout_out INT 0
CONFIG proxy.config.http.transaction_no_activity_timeout_in INT 30
CONFIG proxy.config.http.transaction_no_activity_timeout_out INT 30

WebSocket Timeouts

Traffic Server provides two configurable timeouts for WebSocket connections. The setting proxy.config.websocket.no_activity_timeout will establish the maximum length of time a stalled WebSocket connection will remain before Traffic Server closes it. proxy.config.websocket.active_timeout sets the maximum duration for all WebSocket connections, regardless of their level of activity.

CONFIG proxy.config.websocket.no_activity_timeout INT 600
CONFIG proxy.config.websocket.active_timeout INT 3600

Network Tuning

Error responses from origins are conistent and costly

If error responses are costly for your origin server to generate, you may elect to have Traffic Server cache these responses for a period of time. The default behavior is to consider all of these responses to be uncacheable, which will lead to every client request to result in an origin request.

This behavior is controlled by both enabling the feature via proxy.config.http.negative_caching_enabled and setting the cache time (in seconds) with proxy.config.http.negative_caching_lifetime.

CONFIG proxy.config.http.negative_caching_enabled INT 1
CONFIG proxy.config.http.negative_caching_lifetime INT 10

Thread Types

Logging Configuration

Plugin Tuning

Common Scenarios and Pitfalls

While environments vary widely and Traffic Server is useful in a great number of different situations, there are at least some recurring elements that may be used as shortcuts to identifying problem areas, or realizing easier performance gains.