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22. DISKD

22.1 What is DISKD?

DISKD refers to some features in Squid-2.4 to improve Disk I/O performance. The basic idea is that each cache_dir has its own diskd child process. The diskd process performs all disk I/O operations (open, close, read, write, unlink) for the cache_dir. Message queues are used to send requests and responses between the Squid and diskd processes. Shared memory is used for chunks of data to be read and written.

22.2 Does it perform better?

Yes. We benchmarked Squid-2.4 with DISKD at the Second IRCache Bake-Off. The results are also described here. At the bakeoff, we got 160 req/sec with diskd. Without diskd, we'd have gotten about 40 req/sec.

22.3 How do I use it?

You need to run Squid version 2.4 or later. Your operating system must support message queues, and shared memory.

To configure Squid for DISKD, use the --enable-storeio option:

% ./configure --enable-storeio=diskd,ufs

22.4 FATAL: Unknown cache_dir type 'diskd'

You didn't put diskd in the list of storeio modules as described above. You need to run configure and and recompile Squid.

22.5 If I use DISKD, do I have to wipe out my current cache?

No. Diskd uses the same storage scheme as the standard "UFS" type. It only changes how I/O is performed.

22.6 How do I configure message queues?

Most Unix operating systems have message queue support by default. One way to check is to see if you have an ipcs command.

However, you will likely need to increase the message queue parameters for Squid. Message queue implementations normally have the following parameters:

MSGMNB

Maximum number of bytes per message queue.

MSGMNI

Maximum number of message queue identifiers (system wide).

MSGSEG

Maximum number of message segments per queue.

MSGSSZ

Size of a message segment.

MSGTQL

Maximum number of messages (system wide).

MSGMAX

Maximum size of a whole message. On some systems you may need to increase this limit. On other systems, you may not be able to change it.

The messages between Squid and diskd are 32 bytes for 32-bit CPUs and 40 bytes for 64-bit CPUs. Thus, MSGSSZ should be 32 or greater. You may want to set it to a larger value, just to be safe.

We'll have two queues for each cache_dir -- one in each direction. So, MSGMNI needs to be at least two times the number of cache_dir's.

I've found that 75 messages per queue is about the limit of decent performance. If each diskd message consists of just one segment (depending on your value of MSGSSZ), then MSGSEG should be greater than 75.

MSGMNB and MSGTQL affect how many messages can be in the queues at one time. Diskd messages shouldn't be more than 40 bytes, but let's use 64 bytes to be safe. MSGMNB should be at least 64*75. I recommend rounding up to the nearest power of two, or 8192.

MSGTQL should be at least 75 times the number of cache_dir's that you'll have.

FreeBSD

Your kernel must have

options         SYSVMSG

You can set the parameters in the kernel as follows. This is just an example. Make sure the values are appropriate for your system:

options         MSGMNB=8192     # max # of bytes in a queue
options         MSGMNI=40       # number of message queue identifiers
options         MSGSEG=512      # number of message segments per queue
options         MSGSSZ=64       # size of a message segment
options         MSGTQL=2048     # max messages in system

Digital Unix

Message queue support seems to be in the kernel by default. Setting the options is as follows:

options         MSGMNB="8192"     # max # bytes on queue
options         MSGMNI="40"       # # of message queue identifiers
options         MSGMAX="2048"     # max message size
options         MSGTQL="2048"     # # of system message headers

by Brenden Phillips

If you have a newer version (DU64), then you can probably use sysconfig instead. To see what the current IPC settings are run

# sysconfig -q ipc
To change them make a file like this called ipc.stanza:
ipc:
        msg-max = 2048
        msg-mni = 40
        msg-tql = 2048
        msg-mnb = 8192
then run
# sysconfigdb -a -f ipc.stanza 
You have to reboot for the change to take effect.

Linux

In my limited browsing on Linux, I didn't see any way to change message queue parameters except to modify the include files and build a new kernel. On my system, the file is /usr/src/linux/include/linux/msg.h.

Stefan Köpsell reports that if you compile sysctl support into your kernel, then you can change the following values:

Solaris

Refer to Demangling Message Queues in Sunworld Magazine.

I don't think the above article really tells you how to set the parameters. You do it in /etc/system with lines like this:

set msgsys:msginfo_msgmax=2048
set msgsys:msginfo_msgmnb=8192
set msgsys:msginfo_msgmni=40
set msgsys:msginfo_msgssz=64
set msgsys:msginfo_msgtql=2048

Of course, you must reboot whenever you modify /etc/system before changes take effect.

22.7 How do I configure shared memory?

Shared memory uses a set of parameters similar to the ones for message queues. The Squid DISKD implementation uses one shared memory area for each cache_dir. Each shared memory area is about 800 kilobytes in size. You may need to modify your system's shared memory parameters:

SHMSEG

Maximum number of shared memory segments per process.

SHMMNI

Maximum number of shared memory segments for the whole system.

SHMMAX

Largest shared memory segment size allowed.

SHMALL

Total amount of shared memory that can be used.

For Squid and DISKD, SHMMNI and SHMMNI must be greater than or equal to the number of cache_dir's that you have. SHMMAX must be at least 800 kilobytes. SHMALL must be at least SHMMAX 800 kilobytes multiplied by the number of cache_dir's.

FreeBSD

Your kernel must have

options         SYSVSHM

You can set the parameters in the kernel as follows. This is just an example. Make sure the values are appropriate for your system:

options         SHMSEG=16       # max shared mem id's per process
options         SHMMNI=32       # max shared mem id's per system
options         SHMMAX=2097152  # max shared memory segment size (bytes)
options         SHMALL=4096     # max amount of shared memory (pages)

Digital Unix

Message queue support seems to be in the kernel by default. Setting the options is as follows:

options         SHMSEG="16"       # max shared mem id's per process
options         SHMMNI="32"       # max shared mem id's per system
options         SHMMAX="2097152"  # max shared memory segment size (bytes)
options         SHMALL=4096       # max amount of shared memory (pages)

by Brenden Phillips

If you have a newer version (DU64), then you can probably use sysconfig instead. To see what the current IPC settings are run

# sysconfig -q ipc
To change them make a file like this called ipc.stanza:
ipc:
        shm-seg = 16
        shm-mni = 32
        shm-max = 2097152
        shm-all = 4096
then run
# sysconfigdb -a -f ipc.stanza 
You have to reboot for the change to take effect.

Linux

In my limited browsing on Linux, I didn't see any way to change shared memory parameters except to modify the include files and build a new kernel. On my system, the file is /usr/src/linux/include/asm-i386/shmparam.h

Oh, it looks like you can change SHMMAX by writing the file /proc/sys/kernel/shmmax.

Stefan Köpsell reports that if you compile sysctl support into your kernel, then you can change the following values:

Solaris

Refer to Shared memory uncovered in Sunworld Magazine.

To set the values, you can put these lines in /etc/system:

set shmsys:shminfo_shmmax=2097152
set shmsys:shminfo_shmmni=32
set shmsys:shminfo_shmseg=16

22.8 Sometimes shared memory and message queues aren't released when Squid exits.

Yes, this is a little problem sometimes. Seems like the operating system gets confused and doesn't always release shared memory and message queue resources when processes exit, especially if they exit abnormally. To fix it you can ``manually'' clear the resources with the ipcs command. Add this command into your RunCache or squid_start script:

ipcs | grep '^[mq]' | awk '{printf "ipcrm -%s %s\n", $1, $2}' | /bin/sh

22.9 What are the Q1 and Q2 parameters?

In the source code, these are called magic1 and magic2. These numbers refer to the number of oustanding requests on a message queue. They are specified on the cache_dir option line, after the L1 and L2 directories:

cache_dir diskd /cache1 1024 16 256 Q1=72 Q2=64

If there are more than Q1 messages outstanding, then Squid will intentionally fail to open disk files for reading and writing. This is a load-shedding mechanism. If your cache gets really really busy and the disks can not keep up, Squid bypasses the disks until the load goes down again.

If there are more than Q2 messages outstanding, then the main Squid process ``blocks'' for a little bit until the diskd process services some of the messages and sends back some replies.

Q1 should be larger than Q2. You want Squid to get to the ``blocking'' condition before it gets to the ``refuse to open files'' condition.

Reasonable values for Q1 and Q2 are 72 and 64, respectively.


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