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8. The Cache Manager

Contributed by Jonathan Larmour <>

8.1 What is the cache manager?

The cache manager (cachemgr.cgi) is a CGI utility for displaying statistics about the squid process as it runs. The cache manager is a convenient way to manage the cache and view statistics without logging into the server.

8.2 How do you set it up?

That depends on which web server you're using. Below you will find instructions for configuring the CERN and Apache servers to permit cachemgr.cgi usage.

EDITOR'S NOTE: readers are encouraged to submit instructions for configuration of cachemgr.cgi on other web server platforms, such as Netscape.

After you edit the server configuration files, you will probably need to either restart your web server or or send it a SIGHUP signal to tell it to re-read its configuration files.

When you're done configuring your web server, you'll connect to the cache manager with a web browser, using a URL such as:

8.3 Cache manager configuration for CERN httpd 3.0

First, you should ensure that only specified workstations can access the cache manager. That is done in your CERN httpd.conf, not in squid.conf.

Protection MGR-PROT { Mask @( }

Wildcards are acceptable, IP addresses are acceptable, and others can be added with a comma-separated list of IP addresses. There are many more ways of protection. Your server documentation has details.

You also need to add: Protect /Squid/* MGR-PROT Exec /Squid/cgi-bin/*.cgi /usr/local/squid/bin/*.cgi This marks the script as executable to those in MGR-PROT.

8.4 Cache manager configuration for Apache

First, make sure the cgi-bin directory you're using is listed with a ScriptAlias in your Apache srm.conf file like this: ScriptAlias /Squid/cgi-bin/ /usr/local/squid/cgi-bin/ It's probably a bad idea to ScriptAlias the entire usr/local/squid/bin/ directory where all the Squid executables live.

Next, you should ensure that only specified workstations can access the cache manager. That is done in your Apache access.conf, not in squid.conf. At the bottom of access.conf file, insert: <Location /Squid/cgi-bin/cachemgr.cgi> order deny,allow deny from all allow from </Location>

You can have more than one allow line, and you can allow domains or networks.

Alternately, cachemgr.cgi can be password-protected. You'd add the following to access.conf:

<Location /Squid/cgi-bin/cachemgr.cgi> AuthUserFile /path/to/password/file AuthGroupFile /dev/null AuthName User/Password Required AuthType Basic <Limit GET> require user cachemanager </Location>

Consult the Apache documentation for information on using htpasswd to set a password for this ``user.''

8.5 Cache manager ACLs in squid.conf

The default cache manager access configuration in squid.conf is:

acl manager proto cache_object acl localhost src acl all src

With the following rules:

http_access deny manager !localhost http_access allow all

The first ACL is the most important as the cache manager program interrogates squid using a special cache_object protocol Try it yourself by doing:

telnet 3128 GET cache_object:// HTTP/1.0

The default ACLs say that if the request is for a cache_object, and it isn't the local host, then deny access; otherwise allow access.

In fact, only allowing localhost access means that on the initial cachemgr.cgi form you can only specify the cache host as localhost. We recommend the following:

acl manager proto cache_object acl localhost src acl example src acl all src

Where is the IP address of your web server. Then modify the rules like this:

http_access deny manager !localhost !example http_access allow all

The default ACLs assume that your web server is on the same machine as squid. Remember that the connection from the cache manager program to squid originates at the web server, not the browser. So if your web server lives somewhere else, you should make sure that IP address of the web server that has cachemgr.cgi installed on it is in the example ACL above.

Always be sure to send a SIGHUP signal to squid any time you change the squid.conf file.

8.6 Why does it say I need a password and a URL?

If you ``drop'' the list box, and browse it, you will see that the password is only required to shutdown the cache, and the URL is required to refresh an object (i.e., retrieve it from its original source again) Otherwise these fields can be left blank: a password is not required to obtain access to the informational aspects of cachemgr.cgi.

8.7 I want to shutdown the cache remotely. What's the password?

See the cachemgr_passwd directive in squid.conf.

8.8 How do I make the cache host default to my cache?

Edit Look at the line HOST_OPT = # -DCACHEMGR_HOSTNAME="getfullhostname()" If the webserver that cachemgr.cgi runs from is the same machine as Squid runs on, just remove the #. If your web server is somewhere else use: HOST_OPT = -DCACHEMGR_HOSTNAME=\"\"

If you change this, you will need to recompile and reinstall cachemgr.cgi before the changes take effect.

8.9 What's the difference between Squid TCP connections and Squid UDP connections?

Browsers and caches use TCP connections to retrieve web objects from web servers or caches. UDP connections are used when another cache using you as a sibling or parent wants to find out if you have an object in your cache that it's looking for. The UDP connections are ICP queries.

8.10 It says the storage expiration will happen in 1970!

Don't worry. The default (and sensible) behavior of squid is to expire an object when it happens to overwrite it. It doesn't explicitly garbage collect (unless you tell it to in other ways).

8.11 What do the Meta Data entries mean?


Entry describing an object in the cache.


An entry in the DNS cache.

Hash link

Link in the cache hash table structure.

URL strings

The strings of the URLs themselves that map to an object number in the cache, allowing access to the StoreEntry.

Basically just like the log file in your cache directory:

  1. PoolMemObject structures
  2. Info about objects currently in memory, (eg, in the process of being transferred).
  3. Pool for Request structures
  4. Information about each request as it happens.
  5. Pool for in-memory object
  6. Space for object data as it is retrieved.

8.12 The pool for in-memory objects is huge, and it doesn't get smaller! Is this a memory leak?

No. This pool only grows, it doesn't shrink. It reflects the largest object cached by squid in its lifetime. If you don't want it to be so large, reduce your cache_mem and object size limits for gopher, http and ftp in squid.conf.

8.13 The ``Total accounted'' field in the meta data isn't the same as the size of my squid!

If it's close to the size mentioned don't worry. If squid is much larger than this field, it is probably a memory leak, and all you can do is watch for new patches and occasionally restart squid.

If squid is much smaller than this field, run for cover! Something is very wrong, and you should probably restart squid.

8.14 In the utilization section, what is Other?

Other is a default category to track objects which don't fall into one of the defined categories.

8.15 In the utilization section, why is the Transfer KB/seccolumn always zero?

This column contains gross estimations of data transfer rates averaged over the entire time the cache has been running. These numbers are unreliable and mostly useless.

8.16 In the utilization section, what is the Object Count?

The number of objects of that type in the cache right now.

8.17 In the utilization section, what is the Max/Current/Min KB?

These refer to the size all the objects of this type have grown to/currently are/shrunk to.

8.18 What is the I/O section about?

These are histograms on the number of bytes read from the network per read(2) call. Somewhat useful for determining maximum buffer sizes.

8.19 What is the Objects section for?

Warning: this will download to your browser a list of every URL in the cache and statistics about it. It can be very, very large. Sometimes it will be larger than the amount of available memory in your client! You probably don't need this information anyway.

8.20 What is the VM Objects section for?

VM Objects are the objects which are in Virtual Memory. These are objects which are currently being retrieved and those which were kept in memory for fast access (accelerator mode).

8.21 What does AVG RTT mean?

Average Round Trip Time. This is how long on average after an ICP ping is sent that a reply is received.

8.22 In the IP cache section, what's the difference between a hit, a negative hit and a miss?

A HIT means that the document was found in the cache. A MISS, that it wasn't found in the cache. A negative hit means that it was found in the cache, but it doesn't exist.

8.23 What do the IP cache contents mean anyway?

The hostname is the name that was requested to be resolved.

For the Flags column:

The TTL column represents ``Time To Live'' (i.e., how long the cache entry is valid). (May be negative if the document has expired.)

The N column is the number of IP addresses from which the cache has documents.

The rest of the line lists all the IP addresses that have been associated with that IP cache entry.

8.24 How do I analyze memory usage from cachemgr.cgi's output?

Look at your cachemgr.cgi Cache Information page. For example: Memory usage for squid via mallinfo(): Total space in arena: 94687 KB Ordinary blocks: 32019 KB 210034 blks Small blocks: 44364 KB 569500 blks Holding blocks: 0 KB 5695 blks Free Small blocks: 6650 KB Free Ordinary blocks: 11652 KB Total in use: 76384 KB 81% Total free: 18302 KB 19% Meta Data: StoreEntry 246043 x 64 bytes = 15377 KB IPCacheEntry 971 x 88 bytes = 83 KB Hash link 2 x 24 bytes = 0 KB URL strings = 11422 KB Pool MemObject structures 514 x 144 bytes = 72 KB ( 70 free) Pool for Request structur 516 x 4380 bytes = 2207 KB ( 2121 free) Pool for in-memory object 6200 x 4096 bytes = 24800 KB ( 22888 free) Pool for disk I/O 242 x 8192 bytes = 1936 KB ( 1888 free) Miscellaneous = 2600 KB total Accounted = 58499 KB

First note that mallinfo() reports 94M in ``arena.'' This is pretty close to what top says (97M).

Of that 94M, 81% (76M) is actually being used at the moment. The rest has been freed, or pre-allocated by malloc(3) and not yet used.

Of the 76M in use, we can account for 58.5M (76%). There are some calls to malloc(3) for which we can't account.

The Meta Data list gives the breakdown of where the accounted memory has gone. 45% has gone to StoreEntry and URL strings. Another 42% has gone to buffering hold objects in VM while they are fetched and relayed to the clients (Pool for in-memory object).

The pool sizes are specified by squid.conf parameters. In version 1.0, these pools are somewhat broken: we keep a stack of unused pages instead of freeing the block. In the Pool for in-memory object, the unused stack size is 1/2 of cache_mem. The Pool for disk IO/ is hardcoded at 200. For MemObject and Request it's 1/8 of your system's FD_SETSIZE value.

If you need to lower your process size, we recommend lowering the max object sizes in the 'http', 'ftp' and 'gopher' config lines. You may also want to lower cache_mem to suit your needs. But if you make cache_mem too low, then some objects may not get saved to disk during high-load periods. Newer Squid versions allow you to set memory_pools off to disable the free memory pools.

8.25 What is the fqdncache and how is it different from the ipcache?

IPCache contains data for the Hostname to IP-Number mapping, and FQDNCache does it the other way round. For example:

IP Cache Contents: Hostname Flags lstref TTL N [IP-Number] C 0 21581 1 C 6 21594 1 C 10 21299 4 ... 2/ DL 1583 -772855 0 Flags: C --> Cached D --> Dispatched N --> Negative Cached L --> Locked lstref: Time since last use TTL: Time-To-Live until information expires N: Count of addresses

FQDN Cache Contents: IP-Number Flags TTL N Hostname C -45570 1 C -58133 1 N -73747 0 Flags: C --> Cached D --> Dispatched N --> Negative Cached L --> Locked TTL: Time-To-Live until information expires N: Count of names

8.26 What does ``Page faults with physical i/o: 4897'' mean?

This question was asked on the squid-users mailing list, to which there were three excellent replies.

From: Jonathan Larmour <>

You get a ``page fault'' when your OS tries to access something in memory which is actually swapped to disk. The term ``page fault'' while correct at the kernel and CPU level, is a bit deceptive to a user, as there's no actual error - this is a normal feature of operation.

Also, this doesn't necessarily mean your squid is swapping by that much. Most operating systems also implement paging for executables, so that only sections of the executable which are actually used are read from disk into memory. Also, whenever squid needs more memory, the fact that the memory was allocated will show up in the page faults.

However, if the number of faults is unusually high, and getting bigger, this could mean that squid is swapping. Another way to verify this is using a program called ``vmstat'' which is found on most UNIX platforms. If you run this as ``vmstat 5'' this will update a display every 5 seconds. This can tell you if the system as a whole is swapping a lot (see your local man page for vmstat for more information).

It is very bad for squid to swap, as every single request will be blocked until the requested data is swapped in. It is better to tweak the cache_mem and/or memory_pools setting in squid.conf, or switch to the NOVM versions of squid, than allow this to happen.

From: Peter Wemm <>

There's two different operations at work, Paging and swapping. Paging is when individual pages are shuffled (either discarded or swapped to/from disk), while ``swapping'' generally means the entire process got sent to/from disk.

Needless to say, swapping a process is a pretty drastic event, and usually only reserved for when there's a memory crunch and paging out cannot free enough memory quickly enough. Also, there's some variation on how swapping is implemented in OS's. Some don't do it at all or do a hybrid of paging and swapping instead.

As you say, paging out doesn't necessarily involve disk IO, eg: text (code) pages are read-only and can simply be discarded if they are not used (and reloaded if/when needed). Data pages are also discarded if unmodified, and paged out if there's been any changes. Allocated memory (malloc) is always saved to disk since there's no executable file to recover the data from. mmap() memory is variable.. If it's backed from a file, it uses the same rules as the data segment of a file - ie: either discarded if unmodified or paged out.

There's also ``demand zeroing'' of pages as well that cause faults.. If you malloc memory and it calls brk()/sbrk() to allocate new pages, the chances are that you are allocated demand zero pages. Ie: the pages are not ``really'' attached to your process yet, but when you access them for the first time, the page fault causes the page to be connected to the process address space and zeroed - this saves unnecessary zeroing of pages that are allocated but never used.

The ``page faults with physical IO'' comes from the OS via getrusage(). It's highly OS dependent on what it means. Generally, it means that the process accessed a page that was not present in memory (for whatever reason) and there was disk access to fetch it. Many OS's load executables by demand paging as well, so the act of starting squid implicitly causes page faults with disk IO - however, many (but not all) OS's use ``read ahead'' and ``prefault'' heuristics to streamline the loading. Some OS's maintain ``intent queues'' so that pages can be selected as pageout candidates ahead of time. When (say) squid touches a freshly allocated demand zero page and one is needed, the OS can page out one of the candidates on the spot, causing a 'fault with physical IO' with demand zeroing of allocated memory which doesn't happen on many other OS's. (The other OS's generally put the process to sleep while the pageout daemon finds a page for it).

The meaning of ``swapping'' varies. On FreeBSD for example, swapping out is implemented as unlocking upages, kernel stack, PTD etc for aggressive pageout with the process. The only thing left of the process in memory is the 'struct proc'. The FreeBSD paging system is highly adaptive and can resort to paging in a way that is equivalent to the traditional swapping style operation (ie: entire process). FreeBSD also tries stealing pages from active processes in order to make space for disk cache. I suspect this is why setting 'memory_pools off' on the non-NOVM squids on FreeBSD is reported to work better - the VM/buffer system could be competing with squid to cache the same pages. It's a pity that squid cannot use mmap() to do file IO on the 4K chunks in it's memory pool (I can see that this is not a simple thing to do though, but that won't stop me wishing. :-).

From: John Line <>

The comments so far have been about what paging/swapping figures mean in a ``traditional'' context, but it's worth bearing in mind that on some systems (Sun's Solaris 2, at least), the virtual memory and filesystem handling are unified and what a user process sees as reading or writing a file, the system simply sees as paging something in from disk or a page being updated so it needs to be paged out. (I suppose you could view it as similar to the operating system memory-mapping the files behind-the-scenes.)

The effect of this is that on Solaris 2, paging figures will also include file I/O. Or rather, the figures from vmstat certainly appear to include file I/O, and I presume (but can't quickly test) that figures such as those quoted by Squid will also include file I/O.

To confirm the above (which represents an impression from what I've read and observed, rather than 100% certain facts...), using an otherwise idle Sun Ultra 1 system system I just tried using cat (small, shouldn't need to page) to copy (a) one file to another, (b) a file to /dev/null, (c) /dev/zero to a file, and (d) /dev/zero to /dev/null (interrupting the last two with control-C after a while!), while watching with vmstat. 300-600 page-ins or page-outs per second when reading or writing a file (rather than a device), essentially zero in other cases (and when not cat-ing).

So ... beware assuming that all systems are similar and that paging figures represent *only* program code and data being shuffled to/from disk - they may also include the work in reading/writing all those files you were accessing...

Ok, so what is unusually high?

You'll probably want to compare the number of page faults to the number of HTTP requests. If this ratio is close to, or exceeding 1, then Squid is paging too much.

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