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8. Memory

8.1 Why does Squid use so much memory!?

Squid uses a lot of memory for performance reasons. It takes much, much longer to read something from disk than it does to read directly from memory.

A small amount of metadata for each cached object is kept in memory. This is the StoreEntry data structure. For Squid-2 this is 56-bytes on "small" pointer architectures (Intel, Sparc, MIPS, etc) and 88-bytes on "large" pointer architectures (Alpha). In addition, There is a 16-byte cache key (MD5 checksum) associated with each StoreEntry. This means there are 72 or 104 bytes of metadata in memory for every object in your cache. A cache with 1,000,000 objects therefore requires 72 MB of memory for metadata only. In practice it requires much more than that.

Squid-1.1 also uses a lot of memory to store in-transit objects. This version stores incoming objects only in memory, until the transfer is complete. At that point it decides whether or not to store the object on disk. This means that when users download large files, your memory usage will increase significantly. The squid.conf parameter maximum_object_size determines how much memory an in-transit object can consume before we mark it as uncachable. When an object is marked uncachable, there is no need to keep all of the object in memory, so the memory is freed for the part of the object which has already been written to the client. In other words, lowering maximum_object_size also lowers Squid-1.1 memory usage.

Other uses of memory by Squid include:

8.2 How can I tell how much memory my Squid process is using?

One way is to simply look at ps output on your system. For BSD-ish systems, you probably want to use the -u option and look at the VSZ and RSS fields:

    wessels ~ 236% ps -axuhm
    squid     9631  4.6 26.4 141204 137852  ??  S    10:13PM   78:22.80 squid -NCYs
For SYSV-ish, you probably want to use the -l option. When interpreting the ps output, be sure to check your ps manual page. It may not be obvious if the reported numbers are kbytes, or pages (usually 4 kb).

A nicer way to check the memory usage is with a program called top:

            last pid: 20128;  load averages:  0.06,  0.12,  0.11                   14:10:58
    46 processes:  1 running, 45 sleeping
    CPU states:     % user,     % nice,     % system,     % interrupt,     % idle
    Mem: 187M Active, 1884K Inact, 45M Wired, 268M Cache, 8351K Buf, 1296K Free
    Swap: 1024M Total, 256K Used, 1024M Free

     9631 squid     2   0   138M   135M select  78:45  3.93%  3.93% squid

Finally, you can ask the Squid process to report its own memory usage. This is available on the Cache Manager info page. Your output may vary depending upon your operating system and Squid version, but it looks similar to this:

    Resource usage for squid:
            Maximum Resident Size: 137892 KB
    Memory usage for squid via mstats():
            Total space in arena:  140144 KB
            Total free:              8153 KB 6%

If your RSS (Resident Set Size) value is much lower than your process size, then your cache performance is most likely suffering due to paging.

8.3 My Squid process grows without bounds.

You might just have your cache_mem parameter set too high. See the `` What can I do to reduce Squid's memory usage?'' entry below.

When a process continually grows in size, without levelling off or slowing down, it often indicates a memory leak. A memory leak is when some chunk of memory is used, but not free'd when it is done being used.

Memory leaks are a real problem for programs (like Squid) which do all of their processing within a single process. Historically, Squid has had real memory leak problems. But as the software has matured, we believe almost all of Squid's memory leaks have been eliminated, and new ones are least easy to identify.

Memory leaks may also be present in your system's libraries, such as libc.a or even libmalloc.a. If you experience the ever-growing process size phenomenon, we suggest you first try an alternative malloc library.

8.4 I set cache_mem to XX, but the process grows beyond that!

The cache_mem parameter does NOT specify the maximum size of the process. It only specifies how much memory to use for caching ``hot'' (very popular) replies. Squid's actual memory usage is depends very strongly on your cache size (disk space) and your incoming request load. Reducing cache_mem will usually also reduce the process size, but not necessarily, and there are other ways to reduce Squid's memory usage (see below).

See also How much memory do I need in my Squid server?.

8.5 How do I analyze memory usage from the cache manger output?

Note: This information is specific to Squid-1.1 versions

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 I/O 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.

8.6 The ``Total memory accounted'' value is less than the size of my Squid process.

We are not able to account for all memory that Squid uses. This would require excessive amounts of code to keep track of every last byte. We do our best to account for the major uses of memory.

Also, note that the malloc and free functions have their own overhead. Some additional memory is required to keep track of which chunks are in use, and which are free. Additionally, most operating systems do not allow processes to shrink in size. When a process gives up memory by calling free, the total process size does not shrink. So the process size really represents the maximum size your Squid process has reached.

8.7 xmalloc: Unable to allocate 4096 bytes!

by Henrik Nordstrom

Messages like "FATAL: xcalloc: Unable to allocate 4096 blocks of 1 bytes!" appear when Squid can't allocate more memory, and on most operating systems (inclusive BSD) there are only two possible reasons:

  1. The machine is out of swap
  2. The process' maximum data segment size has been reached
The first case is detected using the normal swap monitoring tools available on the platform (pstat on SunOS, perhaps pstat is used on BSD as well).

To tell if it is the second case, first rule out the first case and then monitor the size of the Squid process. If it dies at a certain size with plenty of swap left then the max data segment size is reached without no doubts.

The data segment size can be limited by two factors:

  1. Kernel imposed maximum, which no user can go above
  2. The size set with ulimit, which the user can control.

When squid starts it sets data and file ulimit's to the hard level. If you manually tune ulimit before starting Squid make sure that you set the hard limit and not only the soft limit (the default operation of ulimit is to only change the soft limit). root is allowed to raise the soft limit above the hard limit.

This command prints the hard limits:

        ulimit -aH

This command sets the data size to unlimited:

        ulimit -HSd unlimited


by Arjan de Vet

The default kernel limit on BSD/OS for datasize is 64MB (at least on 3.0 which I'm using).

Recompile a kernel with larger datasize settings:

        maxusers        128
        # Support for large inpcb hash tables, e.g. busy WEB servers.
        options         INET_SERVER
        # support for large routing tables, e.g. gated with full Internet routing:
        options         "KMEMSIZE=\(16*1024*1024\)"
        options         "DFLDSIZ=\(128*1024*1024\)"
        options         "DFLSSIZ=\(8*1024*1024\)"
        options         "SOMAXCONN=128"
        options         "MAXDSIZ=\(256*1024*1024\)"

See /usr/share/doc/bsdi/config.n for more info.

In /etc/login.conf I have this:

                :path=/bin /usr/bin /usr/contrib/bin:\

        # Settings used by /etc/rc and root
        # This must be set properly for daemons started as root by inetd as well.
        # Be sure reset these values back to system defaults in the default class!
                :path=/bin /usr/bin /sbin /usr/sbin:\
        #       :datasize-cur=128M:\
        #       :openfiles-cur=256:\
        #       :maxproc-cur=256:\

This should give enough space for a 256MB squid process.

FreeBSD (2.2.X)

by Duane Wessels

The procedure is almost identical to that for BSD/OS above. Increase the open filedescriptor limit in /sys/conf/param.c:

        int     maxfiles = 4096;
        int     maxfilesperproc = 1024;
Increase the maximum and default data segment size in your kernel config file, e.g. /sys/conf/i386/CONFIG:
        options         "MAXDSIZ=(512*1024*1024)"
        options         "DFLDSIZ=(128*1024*1024)"
We also found it necessary to increase the number of mbuf clusters:
        options         "NMBCLUSTERS=10240"
And, if you have more than 256 MB of physical memory, you probably have to disable BOUNCE_BUFFERS (whatever that is), so comment out this line:
        #options        BOUNCE_BUFFERS          #include support for DMA bounce buffers

Also, update limits in /etc/login.conf:

        # Settings used by /etc/rc
And don't forget to run ``cap_mkdb /etc/login.conf'' after editing that file.

OSF, Digital Unix

by Ong Beng Hui

To increase the data size for Digital UNIX, edit the file /etc/sysconfigtab and add the entry...

Or, with csh, use the limit command, such as
        > limit datasize 1024M

Editing /etc/sysconfigtab requires a reboot, but the limit command doesn't.

8.8 fork: (12) Cannot allocate memory

When Squid is reconfigured (SIGHUP) or the logs are rotated (SIGUSR1), some of the helper processes (dnsserver) must be killed and restarted. If your system does not have enough virtual memory, the Squid process may not be able to fork to start the new helper processes. This is due to the UNIX way of starting child processes using the fork() system call which temporary duplicates the whole Squid process, and when rapidly starting many child processes such as on "squid -k rotate" the memory usage can temporarily grow to many times the normal memory usage due to several temporary copies of the whole process.

The best way to fix this is to increase your virtual memory by adding swap space. Normally your system uses raw disk partitions for swap space, but most operating systems also support swapping on regular files (Digital Unix excepted). See your system manual pages for swap, swapon, and mkfile. Alternatively you can use the sleep_after_fork directive to make Squid sleep a little while invoking helpers to allow the helper to start up before trying to start the next one. This can be helpful if you find that Squid sometimes fail to restart all helpers on "squid -k reconfigure".

8.9 What can I do to reduce Squid's memory usage?

If your cache performance is suffering because of memory limitations, you might consider buying more memory. But if that is not an option, There are a number of things to try:

8.10 Using an alternate malloc library.

Many users have found improved performance and memory utilization when linking Squid with an external malloc library. We recommend either GNU malloc, or dlmalloc.

Using GNU malloc

To make Squid use GNU malloc follow these simple steps:

  1. Download the GNU malloc source, available from one of The GNU FTP Mirror sites.
  2. Compile GNU malloc
            % gzip -dc malloc.tar.gz | tar xf -
            % cd malloc
            % vi Makefile     # edit as needed
            % make
  3. Copy libmalloc.a to your system's library directory and be sure to name it libgnumalloc.a.
            % su
            # cp malloc.a /usr/lib/libgnumalloc.a
  4. (Optional) Copy the GNU malloc.h to your system's include directory and be sure to name it gnumalloc.h. This step is not required, but if you do this, then Squid will be able to use the mstat() function to report memory usage statistics on the cachemgr info page.
            # cp malloc.h /usr/include/gnumalloc.h
  5. Reconfigure and recompile Squid
            % make realclean
            % ./configure ...
            % make
            % make install
    Note, In later distributions, 'realclean' has been changed to 'distclean'. As the configure script runs, watch its output. You should find that it locates libgnumalloc.a and optionally gnumalloc.h.


dlmalloc has been written by Doug Lea. According to Doug:

This is not the fastest, most space-conserving, most portable, or most tunable malloc ever written. However it is among the fastest while also being among the most space-conserving, portable and tunable.

dlmalloc is included with the Squid-2 source distribution. To use this library, you simply give an option to the configure script:

        % ./configure --enable-dlmalloc ...

8.11 How much memory do I need in my Squid server?

As a rule of thumb on Squid uses approximately 10 MB of RAM per GB of the total of all cache_dirs (more on 64 bit servers such as Alpha), plus your cache_mem setting and about an additional 10-20MB. It is recommended to have at least twice this amount of physical RAM available on your Squid server. For a more detailed discussion on Squid's memory usage see the sections above.

The recommended extra RAM besides what is used by Squid is used by the operating system to improve disk I/O performance and by other applications or services running on the server. This will be true even of a server which runs Squid as the only tcp service, since there is a minimum level of memory needed for process management, logging, and other OS level routines.

If you have a low memory server, and a large disk, then you will not necessarily be able to use all the disk space, since as the cache fills the memory available will be insufficient, forcing Squid to swap out memory and affecting performance. A very large cache_dir total and insufficient physical RAM + Swap could cause Squid to stop functioning completely. The solution for larger caches is to get more physical RAM; allocating more to Squid via cache_mem will not help.

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