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I'm running:

Ubuntu 10.04 LTS Server AMD64 in a 512MB XEN paravirtualized environment.
lighttpd v1.4.26
PHP v5.3.8
MySQL v5.1.41-3ubuntu12

The issue is:

I first started on a '256MB account' which is a lie as it registers as 245MB using free -m I found that after I got the system up and running, the memory started slowly disappearing which I later learned was the disk to memory cache/buffer but it still made me nervous as it looked like a memory leak. Sure enough, when it reached the max memory the server crashed. I thought that it was maybe something wrong with my code or that I was using a buggy version of the new PHP. So i changed my cloud size from 256 to 512, which is also a lie, as it reports as 496MB and let that run. After a week, the memory filled up again and the server crashed.

For reference I am not using drupal or php-nuke or anything pre-bundled or bloated. I tried force upgrading from the maximum PHP v5.3.2 LTS release to 5.3.8 My swappiness is set to the default 60.

here is my memory as it sits now:

        total   used    free    shared  buffers cached
Mem:    496     187     308     0       32      65
-/+     buffers/cache:  89      406
Swap:   1023    0       1023

The site stays at the 89MB level almost always, but the buffers and cached keep raising. my workaround is I created a daily cron with echo 3 > /proc/sys/vm/drop_caches. This has worked for 3 weeks now but I'm worried that once the site goes mainstream that this hack method will fail. I am here to ask you guys out there that know much more than I do about this scenario, what should I do next?

I'm totally willing to get any data you need to help diagnose this.

My personal hunch is an incompatibility with XEN and Ubuntu, as my guess is XEN says there is a real 512MB of memory to use when its only 496, or that XEN says there is 16GB of memory to use as its incorrect proportioning of the VM. No idea how to confirm that.

sysadmin1138
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Kae Cyphet
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2 Answers2

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Why would you drop buffers and caches? Free memory is wasted memory - all the modern operating systems, including Linux, do aggressively cache things like directory paths, actively used files and so on in RAM, as accessing RAM is insanely fast compared to hard disk.

If some application suddenly needs RAM, cached RAM will be immediately freed for that application. This is a very fast operation and does not cause notable performance hit.

So in Linux you actually calculate your free RAM with total - (free + buffers + cached).

Janne Pikkarainen
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My hunch was right on! It seems that the pre-built PV image i chose did not properly create a swap partition that Linux could identify and use. So after the file cache filled up all the memory, the system would go to cache and either it acted as if there was 0MB swap or it panicked and froze in sort of a cache/swap loop.

USE fdisk -l

here are the examples of different file systems i researched. (PV stands for Paravirtualized, HVM means Fully Virtualized, in a XEN environment)

Custom/Clean HVM Install via CDROM-ISO:

Disk /dev/sda: 16.1 GB, 16106127360 bytes
255 heads, 63 sectors/track, 1958 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0001d37d

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1        1871    15021056   83  Linux
/dev/sda2            1871        1958      704513    5  Extended
/dev/sda5            1871        1958      704512   82  Linux swap / Solaris

Pre built PV/HVM:

Disk /dev/sda: 16.1 GB, 16106127360 bytes
255 heads, 63 sectors/track, 1958 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00084eb7

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13       96256   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              13          75      499712   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3              75        1958    15130643   83  Linux

Failed PV:

Disk /dev/sda1: 20.4 GB, 20401094656 bytes
255 heads, 63 sectors/track, 2480 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/sda1 doesn't contain a valid partition table

Disk /dev/sda2: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/sda2 doesn't contain a valid partition table
Kae Cyphet
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