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9. The Cache Manager

by Jonathan Larmour

9.1 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.

9.2 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:

        http://www.example.com/Squid/cgi-bin/cachemgr.cgi/

9.3 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    @(workstation.example.com)
        }

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.

9.4 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 workstation.example.com
        </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
        require user cachemanager
        </Location>

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

9.5 Cache manager configuration for Roxen 2.0 and later

by Francesco ``kinkie'' Chemolli

Notice: this is not how things would get best done with Roxen, but this what you need to do go adhere to the example. Also, knowledge of basic Roxen configuration is required.

This is what's required to start up a fresh Virtual Server, only serving the cache manager. If you already have some Virtual Server you wish to use to host the Cache Manager, just add a new CGI support module to it.

Create a new virtual server, and set it to host http://www.example.com/. Add to it at least the following modules:

  • Content Types
  • CGI scripting support

In the CGI scripting support module, section Settings, change the following settings:

  • CGI-bin path: set to /Squid/cgi-bin/
  • Handle *.cgi: set to no
  • Run user scripts as owner: set to no
  • Search path: set to the directory containing the cachemgr.cgi file

In section Security, set Patterns to:

allow ip=1.2.3.4
where 1.2.3.4 is the IP address for workstation.example.com

Save the configuration, and you're done.

9.6 Cache manager ACLs in squid.conf

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

        acl manager proto cache_object
        acl localhost src 127.0.0.1/255.255.255.255
        acl all src 0.0.0.0/0.0.0.0

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 mycache.example.com 3128
        GET cache_object://mycache.example.com/info 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 127.0.0.1/255.255.255.255
        acl example src 123.123.123.123/255.255.255.255
        acl all src 0.0.0.0/0.0.0.0

Where 123.123.123.123 is the IP address of your web server. Then modify the rules like this:

        http_access allow manager localhost
        http_access allow manager example
        http_access deny manager
        http_access allow all
If you're using miss_access, then don't forget to also add a miss_access rule for the cache manager:
        miss_access allow manager

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.

9.7 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.

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

See the cachemgr_passwd directive in squid.conf.

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

When you run configure use the --enable-cachemgr-hostname option:

% ./configure --enable-cachemgr-hostname=`hostname` ...

Note, if you do this after you already installed Squid before, you need to make sure cachemgr.cgi gets recompiled. For example:

% cd src
% rm cachemgr.o cachemgr.cgi
% make cachemgr.cgi

Then copy cachemgr.cgi to your HTTP server's cgi-bin directory.

9.10 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.

9.11 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).

9.12 What do the Meta Data entries mean?

StoreEntry

Entry describing an object in the cache.

IPCacheEntry

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.

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

9.13 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.

9.14 In the utilization section, why is the Transfer KB/sec column 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.

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

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

9.16 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.

9.17 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.

9.18 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.

9.19 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).

9.20 Что значит AVG RTT?

Среднее время отклика. Указывает как долго (в среднем) приходит ответ после посылки ICP ping.

9.21 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.

9.22 What do the IP cache contents mean anyway?

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

For the Flags column:

  • C Means positively cached.
  • N Means negatively cached.
  • P Means the request is pending being dispatched.
  • D Means the request has been dispatched and we're waiting for an answer.
  • L Means it is a locked entry because it represents a parent or sibling.

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.

9.23 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]
        gorn.cc.fh-lippe.de               C       0  21581 1 193.16.112.73
        lagrange.uni-paderborn.de         C       6  21594 1 131.234.128.245
        www.altavista.digital.com         C      10  21299 4 204.123.2.75  ...
        2/ftp.symantec.com                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
        130.149.17.15                    C -45570 1 andele.cs.tu-berlin.de
        194.77.122.18                    C -58133 1 komet.teuto.de
        206.155.117.51                   N -73747 0

        Flags:  C --> Cached
                D --> Dispatched
                N --> Negative Cached
                L --> Locked
        TTL:    Time-To-Live until information expires
        N:      Count of names

9.24 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.

by 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.

by 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. :-).

by 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.

9.25 What does the IGNORED field mean in the 'cache server list'?

This refers to ICP replies which Squid ignored, for one of these reasons:

  • The URL in the reply could not be found in the cache at all.
  • The URL in the reply was already being fetched. Probably this ICP reply arrived too late.
  • The URL in the reply did not have a MemObject associated with it. Either the request is already finished, or the user aborted before the ICP arrived.
  • The reply came from a multicast-responder, but the cache_peer_access configuration does not allow us to forward this request to that neighbor.
  • Source-Echo replies from known neighbors are ignored.
  • ICP_OP_DENIED replies are ignored after the first 100.


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