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Community ▶ Questions ▶ Is it normal to have half Swap used?
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Is it normal to have half Swap used?

development

This is the result of running command 'free -m' on my system (CentOS 5 + Lighttpd + PHP 5.26):

> total used free shared buffers cached

Mem: 360 353 6 0 5 109

-/+ buffers/cache: 238 121

Swap: 255 124 131

You can see that half of the Swap is being used. I checked several times every day and the range is about 99 - 130 (Swap).

I wonder if this is normal?

5 Replies

Well, it's normal if your processes don't fit into RAM. :)

I think you'll want to post at least some 'ps' output if you need help diagnosing your memory usage.

here's the output for ps -e:

> PID TTY TIME CMD

1 ? 00:00:00 init

2 ? 00:00:00 migration/0

3 ? 00:00:00 ksoftirqd/0

4 ? 00:00:00 migration/1

5 ? 00:00:00 ksoftirqd/1

6 ? 00:00:00 migration/2

7 ? 00:00:00 ksoftirqd/2

8 ? 00:00:00 migration/3

9 ? 00:00:00 ksoftirqd/3

10 ? 00:00:00 events/0

11 ? 00:00:00 events/1

12 ? 00:00:00 events/2

13 ? 00:00:00 events/3

14 ? 00:00:00 khelper

15 ? 00:00:00 kthread

17 ? 00:00:00 xenwatch

18 ? 00:00:00 xenbus

27 ? 00:00:00 kblockd/0

28 ? 00:00:00 kblockd/1

29 ? 00:00:00 kblockd/2

30 ? 00:00:00 kblockd/3

31 ? 00:00:00 cqueue/0

32 ? 00:00:00 cqueue/1

33 ? 00:00:00 cqueue/2

34 ? 00:00:00 cqueue/3

36 ? 00:00:00 kseriod

110 ? 00:00:06 kswapd0

111 ? 00:00:00 aio/0

112 ? 00:00:00 aio/1

113 ? 00:00:00 aio/2

114 ? 00:00:00 aio/3

116 ? 00:00:00 jfsIO

117 ? 00:00:00 jfsCommit

118 ? 00:00:00 jfsCommit

119 ? 00:00:00 jfsCommit

120 ? 00:00:00 jfsCommit

121 ? 00:00:00 jfsSync

122 ? 00:00:00 xfslogd/0

123 ? 00:00:00 xfslogd/1

124 ? 00:00:00 xfslogd/2

125 ? 00:00:00 xfslogd/3

126 ? 00:00:00 xfsdatad/0

127 ? 00:00:00 xfsdatad/1

128 ? 00:00:00 xfsdatad/2

129 ? 00:00:00 xfsdatad/3

738 ? 00:00:00 net_accel/0

739 ? 00:00:00 net_accel/1

740 ? 00:00:00 net_accel/2

741 ? 00:00:00 net_accel/3

748 ? 00:00:00 kpsmoused

752 ? 00:00:00 kcryptd/0

753 ? 00:00:00 kcryptd/1

754 ? 00:00:00 kcryptd/2

755 ? 00:00:00 kcryptd/3

756 ? 00:00:00 kmirrord

765 ? 00:00:03 kjournald

800 ? 00:00:00 kauditd

834 ? 00:00:00 udevd

2221 ? 00:00:00 dhclient

2296 ? 00:00:00 syslogd

2299 ? 00:00:00 klogd

2446 ? 00:00:00 crond

2467 ? 00:00:00 atd

2473 tty1 00:00:00 mingetty

13661 ? 00:00:00 proftpd

13909 ? 00:00:00 sshd

5924 ? 00:00:00 master

5928 ? 00:00:00 qmgr

5935 ? 00:00:00 saslauthd

5936 ? 00:00:00 saslauthd

5937 ? 00:00:00 saslauthd

5938 ? 00:00:00 saslauthd

5940 ? 00:00:00 saslauthd

5948 ? 00:00:00 dovecot

5950 ? 00:00:00 dovecot-auth

5952 ? 00:00:00 pop3-login

5953 ? 00:00:00 pop3-login

5954 ? 00:00:00 pop3-login

5956 ? 00:00:00 imap-login

5957 ? 00:00:00 imap-login

6011 ? 00:00:00 tlsmgr

8829 ? 00:00:00 screen

8830 pts/2 00:00:00 bash

8840 pts/2 00:00:00 su

8841 pts/2 00:00:00 bash

8861 pts/2 00:00:00 su

8862 pts/2 00:00:00 bash

9175 pts/2 00:00:00 su

9176 pts/2 00:00:00 bash

11166 ? 00:00:00 imap-login

12998 ? 00:00:00 pdflush

17719 ? 00:00:00 mysqld_safe

17763 ? 00:00:48 mysqld

5063 ? 00:00:00 pdflush

6754 ? 00:00:00 lighttpd

6755 ? 00:00:00 php-cgi

6757 ? 00:00:00 php-cgi

6758 ? 00:00:00 php-cgi

6759 ? 00:00:00 php-cgi

6760 ? 00:00:00 php-cgi

6761 ? 00:00:00 php-cgi

6762 ? 00:00:00 php-cgi

6763 ? 00:00:00 php-cgi

6764 ? 00:00:00 php-cgi

6765 ? 00:00:00 php-cgi

6766 ? 00:00:00 php-cgi

6767 ? 00:00:00 php-cgi

6768 ? 00:00:00 php-cgi

6769 ? 00:00:00 php-cgi

6770 ? 00:00:00 php-cgi

6771 ? 00:00:00 php-cgi

6772 ? 00:00:00 php-cgi

6773 ? 00:00:00 php-cgi

6774 ? 00:00:02 php-cgi

6775 ? 00:00:02 php-cgi

6776 ? 00:00:03 php-cgi

6777 ? 00:00:02 php-cgi

6778 ? 00:00:03 php-cgi

6779 ? 00:00:02 php-cgi

6780 ? 00:00:02 php-cgi

6781 ? 00:00:02 php-cgi

6782 ? 00:00:02 php-cgi

6783 ? 00:00:02 php-cgi

6784 ? 00:00:03 php-cgi

6785 ? 00:00:02 php-cgi

6786 ? 00:00:02 php-cgi

6787 ? 00:00:02 php-cgi

6788 ? 00:00:04 php-cgi

6789 ? 00:00:03 php-cgi

7792 ? 00:00:00 pickup

9115 ? 00:00:00 sshd

9117 ? 00:00:00 sshd

9118 pts/0 00:00:00 bash

9138 pts/0 00:00:00 su

9139 pts/0 00:00:00 bash

9269 pts/0 00:00:00 ps

I have no idea what that means though :(

Well, you certainly have a lot of php-cgi processes running for that little amount of RAM. MySQL is also a big memory eater. You're definitely being ambitious for a linode of that size.

You can run 'man ps' to view the ps command manual. It's a complicated but powerful command.

A 'ps aux' will show us approximate memory usage per process, but I think it's fairly obvious that this is just a case of too much running at once. See if stopping mysql and then lighttpd (separately) improves the situation.

The normal steps at this point are to 1) run fewer processes 2) scale back resources used by the processes you do need. As I recall, there are a few good articles in the wiki about dealing with this.

You're definitely pushing the limits of what you should be trying to do on a Linode 360, but you're not necessarily going too far yet. The actual amount of swap space you're using isn't what's important–it's how much data you're actually swapping in and out over a given period of time. You can use the vmstat utility to watch swap usage over time to get a better idea of what's going on. Below is the output of vmstat on my Linode 360 for six samples over a one minute period: mt-elbert ~ $ vmstat 10 procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 84 42764 45300 87132 0 0 0 1 1 0 0 0 100 0 0 0 84 42764 45300 87132 0 0 0 0 110 211 0 0 100 0 0 0 84 42764 45300 87132 0 0 0 0 108 211 0 0 100 0 0 0 84 42764 45300 87132 0 0 0 1 110 214 0 0 100 0 0 0 84 42764 45300 87132 0 0 0 0 108 211 0 0 100 0 0 0 84 42764 45300 87132 0 0 0 0 107 211 0 0 100 0 0 0 84 42764 45300 87132 0 0 0 0 110 212 0 0 100 0 The first line when you run vmstat is an average since boot, and the following lines are the number of operations occurring since the last sample. As you can see, my average swap i/o since boot is zero. This is good. If, however, you're seeing a high average, or if the swap operations spike significantly during periods of high load, you probably need to free up memory by running less processes or upgrading your Linode.

There are several tools available to monitor and graph your swap i/o over time that will help you determine if there is a problem that needs to be addressed. From what I've seen, Munin is pretty good for getting detailed data without too much setup hassle, but I haven't used it in a while. YMMV.

So, having said all of that, what really matters is monitoring your system over time to look for trends. Things like Munin are awesome for that as they allow you to see trends very easily. The trends you want to look for on a Linode, obviously, are things that cause disk i/o to spike since that is your most expensive resource from a performance perspective.

–James

Thanks to James and Matt!! appreciate it.

I only have 6 low-traffic websites hosted on my Linode though.

Below is my output of vmstat:

li27-127 ~: vmstat
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  0 141464  18520   2024  96432    0    0     5    12   13   17  0  0 100  0  0

Do the i/o values look all right? I'll try Munin for monitoring my Linode 360.

Below is the output of free -m:

li27-127 ~: free -m
             total       used       free     shared    buffers     cached
Mem:           360        342         17          0          2         94
-/+ buffers/cache:        246        113
Swap:          255        138        117

The output of ps axu is:

li27-127 ~: ps axu
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1   2068   520 ?        Ss   Aug24   0:00 init [3]
root         2  0.0  0.0      0     0 ?        S    Aug24   0:00 [migration/0]
root         3  0.0  0.0      0     0 ?        SN   Aug24   0:00 [ksoftirqd/0]
root         4  0.0  0.0      0     0 ?        S    Aug24   0:00 [migration/1]
root         5  0.0  0.0      0     0 ?        SN   Aug24   0:00 [ksoftirqd/1]
root         6  0.0  0.0      0     0 ?        S    Aug24   0:00 [migration/2]
root         7  0.0  0.0      0     0 ?        SN   Aug24   0:00 [ksoftirqd/2]
root         8  0.0  0.0      0     0 ?        S    Aug24   0:00 [migration/3]
root         9  0.0  0.0      0     0 ?        SN   Aug24   0:00 [ksoftirqd/3]
root        10  0.0  0.0      0     0 ?        S<   Aug24   0:00 [events/0]
root        11  0.0  0.0      0     0 ?        S<   Aug24   0:00 [events/1]
root        12  0.0  0.0      0     0 ?        S<   Aug24   0:00 [events/2]
root        13  0.0  0.0      0     0 ?        S<   Aug24   0:00 [events/3]
root        14  0.0  0.0      0     0 ?        S<   Aug24   0:00 [khelper]
root        15  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kthread]
root        17  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xenwatch]
root        18  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xenbus]
root        27  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kblockd/0]
root        28  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kblockd/1]
root        29  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kblockd/2]
root        30  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kblockd/3]
root        31  0.0  0.0      0     0 ?        S<   Aug24   0:00 [cqueue/0]
root        32  0.0  0.0      0     0 ?        S<   Aug24   0:00 [cqueue/1]
root        33  0.0  0.0      0     0 ?        S<   Aug24   0:00 [cqueue/2]
root        34  0.0  0.0      0     0 ?        S<   Aug24   0:00 [cqueue/3]
root        36  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kseriod]
root       110  0.0  0.0      0     0 ?        S<   Aug24   0:07 [kswapd0]
root       111  0.0  0.0      0     0 ?        S<   Aug24   0:00 [aio/0]
root       112  0.0  0.0      0     0 ?        S<   Aug24   0:00 [aio/1]
root       113  0.0  0.0      0     0 ?        S<   Aug24   0:00 [aio/2]
root       114  0.0  0.0      0     0 ?        S<   Aug24   0:00 [aio/3]
root       116  0.0  0.0      0     0 ?        S<   Aug24   0:00 [jfsIO]
root       117  0.0  0.0      0     0 ?        S<   Aug24   0:00 [jfsCommit]
root       118  0.0  0.0      0     0 ?        S<   Aug24   0:00 [jfsCommit]
root       119  0.0  0.0      0     0 ?        S<   Aug24   0:00 [jfsCommit]
root       120  0.0  0.0      0     0 ?        S<   Aug24   0:00 [jfsCommit]
root       121  0.0  0.0      0     0 ?        S<   Aug24   0:00 [jfsSync]
root       122  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfslogd/0]
root       123  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfslogd/1]
root       124  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfslogd/2]
root       125  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfslogd/3]
root       126  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfsdatad/0]
root       127  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfsdatad/1]
root       128  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfsdatad/2]
root       129  0.0  0.0      0     0 ?        S<   Aug24   0:00 [xfsdatad/3]
root       738  0.0  0.0      0     0 ?        S<   Aug24   0:00 [net_accel/0]
root       739  0.0  0.0      0     0 ?        S<   Aug24   0:00 [net_accel/1]
root       740  0.0  0.0      0     0 ?        S<   Aug24   0:00 [net_accel/2]
root       741  0.0  0.0      0     0 ?        S<   Aug24   0:00 [net_accel/3]
root       748  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kpsmoused]
root       752  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kcryptd/0]
root       753  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kcryptd/1]
root       754  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kcryptd/2]
root       755  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kcryptd/3]
root       756  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kmirrord]
root       765  0.0  0.0      0     0 ?        S<   Aug24   0:03 [kjournald]
root       800  0.0  0.0      0     0 ?        S<   Aug24   0:00 [kauditd]
root       834  0.0  0.0   2148   340 ?        S

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