詳細可參考官方資料 http://wiki.squid-cache.org/Features/DelayPools

Feature: Delay Pools

  • Goal: To provide a way to limit the bandwidth of certain requests based on any list of criteria.
  • Status: Completed
  • Version: 2.2+
  • Developer: David Luyer



Delay Pools

by David Luyer.

To enable delay pools features in Squid configure with –enable-delay-pools before compilation.


Terminology for this FAQ entry:

a collection of bucket groups as appropriate to a given class
bucket group
a group of buckets within a pool, such as the per-host bucket group, the per-network bucket group or the aggregate bucket group (the aggregate bucket group is actually a single bucket)
an individual delay bucket represents a traffic allocation which is replenished at a given rate (up to a given limit) and causes traffic to be delayed when empty
the class of a delay pool determines how the delay is applied, ie, whether the different client IPs are treated separately or as a group (or both)
class 1
a class 1 delay pool contains a single unified bucket which is used for all requests from hosts subject to the pool
class 2
a class 2 delay pool contains one unified bucket and 255 buckets, one for each host on an 8-bit network (IPv4 class C)
class 3
contains 255 buckets for the subnets in a 16-bit network, and individual buckets for every host on these networks (IPv4 class B )
class 4
as class 3 but in addition have per authenticated user buckets, one per user.
class 5
custom class based on tag values returned by external_acl_type helpers in http_access. One bucket per used tag value.

Delay pools allows you to limit traffic for clients or client groups, with various features:

  • can specify peer hosts which aren’t affected by delay pools, ie, local peering or other ‘free’ traffic (with theno-delay peer option).
  • delay behavior is selected by ACLs (low and high priority traffic, staff vs students or student vs authenticated student or so on).
  • each group of users has a number of buckets, a bucket has an amount coming into it in a second and a maximum amount it can grow to; when it reaches zero, objects reads are deferred until one of the object’s clients has some traffic allowance.
  • any number of pools can be configured with a given class and any set of limits within the pools can be disabled, for example you might only want to use the aggregate and per-host bucket groups of class 3, not the per-network one.

This allows options such as creating a number of class 1 delay pools and allowing a certain amount of bandwidth to given object types (by using URL regular expressions or similar), and many other uses I’m sure I haven’t even though of beyond the original fair balancing of a relatively small traffic allocation across a large number of users.


There are some limitations of delay pools:

  • delay pools are incompatible with slow aborts; quick abort should be set fairly low to prevent objects being retrieved at full speed once there are no clients requesting them (as the traffic allocation is based on the current clients, and when there are no clients attached to the object there is no way to determine the traffic allocation).
  • delay pools only limits the actual data transferred and is not inclusive of overheads such as TCP overheads, ICP, DNS, ICMP pings, etc.
  • it is possible for one connection or a small number of connections to take all the bandwidth from a given bucket and the other connections to be starved completely, which can be a major problem if there are a number of large objects being transferred and the parameters are set in a way that a few large objects will cause all clients to be starved (potentially fixed by a currently experimental patch).
  • in Squid 3.1 the class-based pools do not work yet with IPv6 addressed clients.
  • In squid older than 3.1 the delay pool bucket is limited to 32-bits and thus has a rather low MB cap on both bucket content and refill rate. The bucket size is now raised to 64-bit ‘unlimited’ values, but refill rate remains low.


How can I limit Squid’s total bandwidth to, say, 512 Kbps?


delay_pools 1
delay_class 1 1
delay_access 1 allow all
delay_parameters 1 64000/64000          # 512 kbits == 64 kbytes per second

The 1 second buffer (max = restore = 64kbytes/sec) is because a limit is requested, and no responsiveness to a burst is requested. If you want it to be able to respond to a burst, increase the aggregate_max to a larger value, and traffic bursts will be handled. It is recommended that the maximum is at least twice the restore value – if there is only a single object being downloaded, sometimes the download rate will fall below the requested throughput as the bucket is not empty when it comes to be replenished.


How to limit a single connection to 128 Kbps?

You can not limit a single HTTP request’s connection speed. You can limit individual hosts to some bandwidth rate. To limit a specific host, define an acl for that host and use the example above. To limit a group of hosts, then you must use a delay pool of class 2 or 3. For example:


acl only128kusers src
delay_pools 1
delay_class 1 3
delay_access 1 allow only128kusers
delay_access 1 deny all
delay_parameters 1 64000/64000 -1/-1 16000/64000

For an explanation of these tags please see the configuration file.

The above gives a solution where a cache is given a total of 512kbits to operate in, and each IP address gets only 128kbits out of that pool.


How do you personally use delay pools?

We have six local cache peers, all with the options ‘proxy-only no-delay’ since they are fast machines connected via a fast ethernet and microwave (ATM) network.

For our local access we use a dstdomain ACL, and for delay pool exceptions we use a dst ACL as well since the delay pool ACL processing is done using “fast lookups”, which means (among other things) it won’t wait for a DNS lookup if it would need one.

Our proxy has two virtual interfaces, one which requires student authentication to connect from machines where a department is not paying for traffic, and one which uses delay pools. Also, users of the main Unix system are allowed to choose slow or fast traffic, but must pay for any traffic they do using the fast cache. Ident lookups are disabled for accesses through the slow cache since they aren’t needed. Slow accesses are delayed using a class 3 delay pool to give fairness between departments as well as between users. We recognize users of Lynx on the main host are grouped together in one delay bucket but they are mostly viewing text pages anyway, so this isn’t considered a serious problem. If it was we could take those hosts into a class 1 delay pool and give it a larger allocation.

I prefer using a slow restore rate and a large maximum rate to give preference to people who are looking at web pages as their individual bucket fills while they are reading, and those downloading large objects are disadvantaged. This depends on which clients you believe are more important. Also, one individual 8 bit network (a residential college) have paid extra to get more bandwidth.

The relevant parts of my configuration file are (IP addresses, etc, all changed):


# ACL definitions
# Local network definitions, domains a.net, b.net
acl LOCAL-NET dstdomain a.net b.net
# Local network; nets 64 - 127.  Also nearby network class A, 10.
acl LOCAL-IP dst
# Virtual i/f used for slow access
acl virtual_slowcache myip
# All permitted slow access, nets 96 - 127
acl slownets src
# Special 'fast' slow access, net 123
acl fast_slow src
# User hosts
acl my_user_hosts src
# Don't need ident lookups for billing on (free) slow cache
ident_lookup_access allow my_user_hosts !virtual_slowcache
ident_lookup_access deny all
# Security access checks
http_access [...]
# These people get in for slow cache access
http_access allow virtual_slowcache slownets
http_access deny virtual_slowcache
# Access checks for main cache
http_access [...]
# Delay definitions (read config file for clarification)
delay_pools 2
delay_initial_bucket_level 50
delay_class 1 3
delay_access 1 allow virtual_slowcache !LOCAL-NET !LOCAL-IP !fast_slow
delay_access 1 deny all
delay_parameters 1 8192/131072 1024/65536 256/32768
delay_class 2 2
delay_access 2 allow virtual_slowcache !LOCAL-NET !LOCAL-IP fast_slow
delay_access 2 deny all
delay_parameters 2 2048/65536 512/32768

The same code is also used by a some of departments using class 2 delay pools to give them more flexibility in giving different performance to different labs or students.


Where else can I find out about delay pools?

This is also pretty well documented in the configuration file, with examples. Squid install with a squid.conf.documented or squid.conf.default file. If you no longer have a documented config file the latest version is provided on the squid-cache.org website.




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