ask for /dev/shm + OpenVZ questions

mcgree

Does it mean that this OpenVZ has a memory overselling.

[email protected]:/dev/shm# free -h
              total        used        free      shared  buff/cache   available
Mem:           6.0G         30M        5.3G         11M        711M        5.9G
Swap:          6.0G          0B        6.0G
[email protected]:/dev/shm# dd if=/dev/zero of=1.bin bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.431965 s, 2.5 GB/s
[email protected]:/dev/shm# free -h
              total        used        free      shared  buff/cache   available
Mem:           6.0G        2.0G        3.3G        1.0G        711M        3.9G
Swap:          6.0G          0B        6.0G
[email protected]:/dev/shm# rm 1.bin 
[email protected]:/dev/shm# dd if=/dev/zero of=1.bin bs=1M count=6144
dd: error writing '1.bin': No space left on device
3073+0 records in
3072+0 records out
3221225472 bytes (3.2 GB, 3.0 GiB) copied, 1.32617 s, 2.4 GB/s
[email protected]:/dev/shm# free -h
              total        used        free      shared  buff/cache   available
Mem:           6.0G        5.3G          0B        3.0G        707M        609M
Swap:          6.0G          0B        6.0G

akb

Default size of /dev/shm is half of your physical memory which is why your dd command couldn't write more than 3GiB.

mcgree

@akb said:
Default size of /dev/shm is half of your physical memory which is why your dd command couldn't write more than 3GiB.

I wrote 1GB to /dev/shm and it's taking up 2GB of memory, why?

mcgree

I just remounted to RAM size, indeed I can write the whole disk, I will study why.

dahartigan

I think /dev/shm must pay the default fine.

akb

@mcgree said: I wrote 1GB to /dev/shm and it's taking up 2GB of memory, why?

The 1GB file that you wrote is showing correctly as 1.0G under "Shared" column. Rest of the used RAM constitutes file system buffers/cache (711M) plus the memory taken by your processes.

mcgree

@akb said:

@mcgree said: I wrote 1GB to /dev/shm and it's taking up 2GB of memory, why?

The 1GB file that you wrote is showing correctly as 1.0G under "Shared" column. Rest of the used RAM constitutes file system buffers/cache (711M) plus the memory taken by your processes.

I understand this description.

yoursunny

I detected massive RAM overselling using a file in shm folder.

1. Create a file in /run/shm.
2. Idle 15 days.
3. Read the file and record throughput.
4. Read the file again and record throughput.

If the file is actually in RAM, both read passes should have the same throughput.

In the paste below, the first read was 75% slower than the second read, which indicates that the first read was not from DRAM.
It might be in PMEM that is still considered RAM, but the processor on this server doesn't support Optane DIMM.
Therefore, I can only conclude the provider is selling disk-based SWAP as RAM,

$ date
Sun 26 Sep 2021 05:17:26 AM CEST
$ ls -l /run/shm
total 512000
-rw-r--r-- 1 sunny sunny 524288000 Aug 23 04:45 random.bin
$ dd if=/run/shm/random.bin of=/dev/null
1024000+0 records in
1024000+0 records out
524288000 bytes (524 MB, 500 MiB) copied, 22.2434 s, 23.6 MB/s
$ dd if=/run/shm/random.bin of=/dev/null
1024000+0 records in
1024000+0 records out
524288000 bytes (524 MB, 500 MiB) copied, 5.32052 s, 98.5 MB/s

Raw paste: Boomer.host sells SWAP as RAM