While the default configuration values for ATS will get you up and running, they're somewhat designed for regression testing and not real-world applications.

This page documents what I've discovered myself through a fair amount of experimentation and real-world experience.

The following lists the steps involved in taking a generic configuration, and modifying it for my own needs. Yours may vary, however, and I'll do my best
to indicate which settings should be sized based on your install.

Please keep in mind the following only applies to creating a forward-only web proxy caching setup.

The following lists the initial steps involved in getting a generic Traffic Server install up and running.

NOTE:  Please use the following with Apache Traffic Server v5.0.0 and higher.

 Home Machine

• Make/Model: Apple iMac Mid 2011
• CPU: 3.4 Ghz Intel Core i7 (quad-core w/Hyperthreading)
• Memory: 16GB
• Disk: 1TB
• OS: Mac OS X v10.9.3
• Cache Size: 8GB
• Browser: Google Chrome v35

Testing Regimen

The following settings have been tested against the following:

• IPv4 websites
• IPv6 websites
• Explicitly difficult web pages (i.e. Bing Image Search)
• Explicitly SSL web sites (i.e. Facebook)
• Internet Radio (HTTP streaming, as well as iTunes Radio & Pandora)

The following settings are all located in /usr/local/etc/trafficserver/records.config.

Since Traffic Server v5.0.0 has reorganized this file, I'll go through the relevant sections here.  When adding configurations, simply add the settings below the existing ones.

Thread Configuration

As I'm using Traffic Server on a personal basis, I decided to explicitly configure it to not consume as many CPU cores as it might do otherwise.

CONFIG proxy.config.exec_thread.autoconfig INT 0
CONFIG proxy.config.exec_thread.limit INT 1

HTTP Connection Timeouts

I'm using Traffic Server on a pretty speedy broadband connection.  As such, I've configured it to be somewhat impatient in terms of timeouts.

Some of these are also borrowed from Mozilla Firefox.

CONFIG proxy.config.http.keep_alive_no_activity_timeout_in INT 600
CONFIG proxy.config.http.keep_alive_no_activity_timeout_out INT 115
CONFIG proxy.config.http.transaction_no_activity_timeout_in INT 10
CONFIG proxy.config.http.transaction_no_activity_timeout_out INT 10
CONFIG proxy.config.http.transaction_active_timeout_in INT 43200
CONFIG proxy.config.http.transaction_active_timeout_out INT 43200
CONFIG proxy.config.http.accept_no_activity_timeout INT 10
CONFIG proxy.config.net.default_inactivity_timeout INT 10

Origin Server Connect Attempts

I had a similar experience tuning Squid in this regard.  This first setting controls how many connections ATS can make outbound to various Internet servers,
on a per-server basis. The default allows for unlimited connections, and while that may be useful on a heavily loaded server I find that it actually slows things down a bit.

I decided to go with 32 connections per origin server simultaneously.

The second setting controls how often Traffic Server will internally poll to process network events.  As I'm on a machine that can't easily handle 2-3% CPU load constantly, I reduced this.

CONFIG proxy.config.http.origin_max_connections 32
CONFIG proxy.config.net.poll_timeout INT 50

Cache Control

The following configurations tell Traffic Server to be more aggressive than it would otherwise, with regard to caching overall as well as some speed-ups.

CONFIG proxy.config.http.chunking.size INT 128K
CONFIG proxy.config.http.server_session_sharing.match STRING host
CONFIG proxy.config.http.cache.ignore_client_no_cache INT 0
CONFIG proxy.config.http.cache.ims_on_client_no_cache INT 0
CONFIG proxy.config.http.cache.ignore_server_no_cache INT 1

Heuristic Cache Expiration

The default config for Traffic Server specifies that after 1 day(86,400 seconds), any object without a specific expiration cannot be cached.

I'd prefer that they stick around for between 1-3 months. This setting is contentious in that what it should be is debatable.

The goal here is to enforce a window of between 1 and 3 months to keep objects in the cache, using Traffic Server's built-in heuristics.

CONFIG proxy.config.http.cache.heuristic_min_lifetime INT 2592000
CONFIG proxy.config.http.cache.heuristic_max_lifetime INT 7776000
CONFIG proxy.config.http.cache.heuristic_lm_factor FLOAT 0.50

RAM And Disk Cache Configuration

The default config for Traffic Server specifies a few things here that can be tuned.

First, I decided to explicitly set my RAM cache settings.

Second, I observed my cache running via the "traffic_top" utility and have set the average object size accordingly.

One should always halve the setting for that configuration, as it allows "headroom" within Traffic Server such that one will never run out of slots in which to store objects.

Third, I've explicitly tuned the average disk fragment setting as well as disabled a feature that for me slows down the cache a bit.

CONFIG proxy.config.cache.ram_cache.size INT 64M
CONFIG proxy.config.cache.ram_cache_cutoff INT 8M
CONFIG proxy.config.cache.min_average_object_size INT 64K
CONFIG proxy.config.cache.target_fragment_size INT 262144
CONFIG proxy.config.cache.enable_read_while_writer INT 0

Logging Configuration

The defaults for Traffic Server specify a squid-compatible logfile that's binary in nature.  I prefer to have the file readable so I'm overriding this.

CONFIG proxy.config.log.squid_log_is_ascii INT 1

Step 1 – Configure

You may or may not wish to enable these configuration options.
They essentially make ATS more aggressive in caching than its default configuration would allow.

HTTP Background Fill Completion

There's an algorithm here that I don't fully understand, but this setting should guarantee that objects
loaded in the background are cached regardless of their size.

From Leif H: "This recommendation is wrong, it should be set to 0.0 for it to always kick in. It allows the server to continue fetching / caching a large object even if the client disconnects. This setting (with a value of 0.0) is a prerequisite for getting read-while-writer to kick in."

I've since updated this setting.

CONFIG proxy.config.http.background_fill_completed_threshold FLOAT 0.000000

HTTP  Cache Options

The default config for ATS specifies that URLs that look dynamic be cached anyways.

Unfortunately, this can break some web applications.  I decided to turn this off.

CONFIG proxy.config.http.cache.cache_urls_that_look_dynamic INT 0

HTTP Cache Options

The default config for ATS specifies that headers for "No-Cache" from the client be ignored, and that those from a server be honored.

I actually prefer the reverse to be true, hoping that the browser knows when bypassing the cache would be useful.

CONFIG proxy.config.http.cache.ignore_client_no_cache INT 0
CONFIG proxy.config.http.cache.ims_on_client_no_cache INT 0
CONFIG proxy.config.http.cache.ignore_server_no_cache INT 1

HTTP Cache Options

The default config for ATS specifies that after 1 day(86,400 seconds), any object without a specific expiration
cannot be cached. I'd prefer that they stick around for between 1-3 months. This setting is contentious in that what it should be is debatable.

From Leif H: "The proxy.config.http.cache.heuristic_max_lifetime is mostly correct, except for two things: 1) It does not get “flushed”, it merely says how long (max) it can cache objects under the heuristics rules. 2) It’s not a fixed setting, what it does is together with the “min” setting is to get a range for the Expire time, the effective TTL will be a value between min-max, based on how old the object is (based on Last-Modified). Since you set lm_factor=1.0, you did effectively set min == max == 3 months. This seems very aggressive, and counteracts how the heuristics system is supposed to work. The idea is that objects which are changing frequently you should cache much shorter than those which change infrequently."

I've since updated these settings. I now believe I'm enforcing a window of between 1 and 3 months
to keep objects in the cache, using ATS's built-in heuristics.

CONFIG proxy.config.http.cache.heuristic_min_lifetime INT 2592000
CONFIG proxy.config.http.cache.heuristic_max_lifetime INT 7776000
CONFIG proxy.config.http.cache.heuristic_lm_factor FLOAT 0.500000

Fuzzy Object Prefetch Logic

From Leif H: "As described here, is not what it does, at all. Fuzzy logic is there to allow (random chance) a client to go to origin before the object is stale in cache. The idea is that you would (for some reasonably active objects) prefetch the object such that you always have it fresh in cache."

An interesting notion, but not one I desire. The following setting disables this feature.

CONFIG proxy.config.http.cache.fuzz.min_time INT 0

Step 2 – Optimize

Accept & Execution Threads

The default config for ATS supports as many CPU cores as you have in your machine.
Typically, ATS will configure 1.5 threads per CPU and automatically scale upwards.

I have 4, but discovered over time that I get a decent performance boost by assigning slightly more threads than ATS would otherwise.

CONFIG proxy.config.exec_thread.autoconfig INT 0
CONFIG proxy.config.exec_thread.limit INT 8
CONFIG proxy.config.accept_threads INT 2

HTTP Chunking

The default config for ATS specifies that the proxy itself use data "chunks" of 4KB each.
Being that I'm on a high-speed Internet link at home, I decided to increase this.
If you find yourself annoyed with how long streaming Internet Radio takes when rebuffering, try setting this to 16KB or 64KB instead.

From what I understand it's a balancing act between what Internet Radio will accept, and throughput of the cache overall.

CONFIG proxy.config.http.chunking.size INT 512K

HTTP Server Sessions

The default config for ATS specifies that connections to origin servers be shared on a per-thread basis.

I found that changing this behavior to a global connection pool made the cache significantly faster.

CONFIG proxy.config.http.share_server_sessions INT 1

HTTP Connection Timeouts

I decided to use some of Mozilla Firefox's values here, along with some of my own.
It turns out proxy.config.http.transaction_active_timeout_in was essentially shutting down my
streaming Internet Radio connections. I increased that setting from 15 minutes to 12 hours.

CONFIG proxy.config.http.keep_alive_no_activity_timeout_in INT 115
CONFIG proxy.config.http.keep_alive_no_activity_timeout_out INT 115
CONFIG proxy.config.http.transaction_no_activity_timeout_in INT 10
CONFIG proxy.config.http.transaction_no_activity_timeout_out INT 10
CONFIG proxy.config.http.transaction_active_timeout_in INT 43200
CONFIG proxy.config.http.transaction_active_timeout_out INT 43200
CONFIG proxy.config.http.accept_no_activity_timeout INT 10

HTTP Origin Server Connections

I had a similar experience tuning Squid in this regard.  This setting controls how many connections ATS can make outbound to various Internet servers,
on a per-server basis. The default allows for unlimited connections, and while that may be
useful on a heavily loaded server I find that it actually slows things down a bit.

I decided to go with 32 connections per origin server simultaneously.

CONFIG proxy.config.http.origin_max_connections 32

HTTP RAM Cache

While the default ATS options for this may be optimal under heavy load,
I found using the simpler LRU algorithm much faster and more useful.

The following specifies 128MB of RAM cache, with objects of up to 8MB in size,
to be managed using LRU. During normal use, RAM utilization should rise and rise
until all 128MB is used, then the LRU algorithm should kick in. Also,
figure on at least 100MB of general RAM overhead for ATS in addition to this.

CONFIG proxy.config.cache.ram_cache.size INT 128M
CONFIG proxy.config.cache.ram_cache_cutoff INT 16M
CONFIG proxy.config.cache.ram_cache.algorithm INT 1
CONFIG proxy.config.cache.ram_cache.use_seen_filter INT 0

NOTE: This setting should be sized relative to the amount of memory you want to use.
Also, it requires restarting ATS to properly take effect.

Disk Target Fragment Size

This setting defines the optimal fragment size when storing objects to disk.  I noticed a considerable performance improvement when changing this from its default.  Please be aware I'm not sure if this setting willl scale properly, but works well for me.

CONFIG proxy.config.cache.target_fragment_size INT 262144

NOTE: This setting requires clearing the disk cache and restarting ATS to properly take effect.

Cache Minimum Average Object Size

This setting is pretty important. It defines a global variable whose function is to both structure the cache
for future objects, as well as optimize other areas.

From Leif H: "Your setting for proxy.config.cache.min_average_object_size seems wrong. If your average object size is 32KB, you should set this to, hem, 32KB . However, to give some headroom, my personal recommendation is to 2x the number of directory entries, so set the configuration to 16KB.

The math is mostly correct, except the calculation for "Disk Cache Object Capacity” is in fact the max number of directory entries the cache can hold. Each object on disk consumes at least one directory entry, but can consume more."

As it turns out, my original thoughts on this were a bit misguided.  For my purposes, I first monitored ATS with the command-line tool "traffic_top".

After letting the cache run for quite some time, I discovered the Average Internet Object Size was much larger than I'd guessed.  As it turns out, my "average internet object" is roughly 80KB in size, and so we can do the following math:

Average Internet Object Size: 80KB

Directory Entries Required Per Object(headroom): 2
Cache Minimum Average Object Size: 40960 (81920 / 2)

CONFIG proxy.config.cache.min_average_object_size INT 40K

NOTE: This setting requires clearing the disk cache and restarting ATS to properly take effect.

Cache Threads Per Disk Spindle

My setting here is somewhat of a rough guess. I've had issues in the past with Squid as a web cache
and increasing the threads dedicated to disk access definitely helped. However, with ATS I've actually
noticed a speed boost by decreasing this setting. My current theory is that this setting should allow
for one thread per CPU core.

CONFIG proxy.config.cache.threads_per_disk INT 4

Cache Disk Write Lock Delay

The default setting for ATS is unfortunately quite low. For some reason this caused a repeatable,
large delay when loading Bing Image Search results. The following setting removes most of the delays
and seems to speed up the cache overall a bit.

From Leif H: "The text around proxy.config.cache.mutex_retry_delay is confusing. Setting this higher would increase latency, not reduce it, at the expense of possibly consuming more CPU. If you experience something different, I think it’d be worthwhile to file a Jira."

I'm sure I have experienced higher latency, but also somehow avoided a disk contention problem.

P.S. Since testing this explicitly, Microsoft Bing has done some backend work that has improved HTTP responses.
I currently believe this setting only really matters when TCP retries come into play.

CONFIG proxy.config.cache.mutex_retry_delay INT 25

Maximum Concurrent DNS Queries

The default settings for ATS regarding DNS are set pretty high. I decided for my purposes to lower them,
Your Milage May Vary on these.

CONFIG proxy.config.dns.max_dns_in_flight INT 512

DNS Internet Protocol Preference

I've no idea if this setting really helps or not, but I like to specify my preference for IPv6 over IPv4
as much as possible.

CONFIG proxy.config.hostdb.ip_resolve STRING ipv6;ipv4

DNS Host Cache Database

The default settings for ATS regarding DNS are set pretty high. I think the following represents a pretty good balance between caching too much and caching too little in terms of DNS.  I've also set the cache to retain DNS records for up to a month, and serve stale records if necessary for up to 5 minutes.  This seems to speed things up a bit.

CONFIG proxy.config.hostdb.size INT 32K
CONFIG proxy.config.hostdb.storage_size INT 8M
CONFIG proxy.config.hostdb.timeout INT 43200
CONFIG proxy.config.hostdb.verify_after 43200
CONFIG proxy.config.hostdb.serve_stale_for 300

HTTP Socket I/O Buffers

The default config for ATS leaves enables some buffering between ATS and the client.  While this isn't necessary, I eventually decided on the following settings.  Combined with other performance tweaks, these work well overall.

CONFIG proxy.config.net.sock_send_buffer_size_in INT 64K
CONFIG proxy.config.net.sock_recv_buffer_size_in INT 8K
CONFIG proxy.config.net.sock_send_buffer_size_out INT 8K
CONFIG proxy.config.net.sock_recv_buffer_size_out INT 64K

Step 3 - Secure

Maximum Inbound Concurrent Connections

The default config for ATS specifies that this server can handle up to 30,000 connections.
For my purposes, that's a bit excessive. I figure with 2,048 connections there's plenty
of "elbow room". Keep in mind this is a global connection limit, so don't forget about
outbound connections!

CONFIG proxy.config.net.connections_throttle INT 2K

That's it. Go ahead and refresh your cache(if necessary), restart ATS, and enjoy your tuned proxy server.