Shared Memory Calculator
Calculate System V or POSIX shared memory segment size requirements for applications.
About Shared Memory Calculator
System V shared memory (used heavily by databases like PostgreSQL for shared_buffers) is bounded by two kernel limits: kernel.shmmax (the largest single segment allowed, in bytes) and kernel.shmall (the total shared memory system-wide, in pages). Applications that request a segment larger than shmmax, or a total exceeding shmall, fail to allocate shared memory at startup — a common cause of database startup failures after increasing configured buffer sizes without also raising these kernel limits.
Formula
shmall in pages
shmall_pages = ceil(total_shm_bytes / page_size)
- B
- total shared memory requested
- p
- system page size
Worked example
With the default values below, Shared Memory Calculator returns Total Shared Memory Needed (MB) of 2,048.00. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Number of Shared Memory Segmentsminimum 1
- 4
- Size per Segment (MB)minimum 0
- 512
- System Page Size (KB)minimum 1
- 4
- Total System RAM (MB)minimum 1
- 16,384
Results
- Total Shared Memory Needed (MB)
- 2,048.00
- As % of Total RAM
- 12.50%
- Recommended kernel.shmmax (bytes)
- 536,870,912
- Recommended kernel.shmall (pages)
- 524,288
- sysctl Command
- sysctl -w kernel.shmmax=536870912 kernel.shmall=524288
Step by step
- 1Values used
- 2shmall in pages
- 3Total Shared Memory Needed (MB)= 2,048.00
- 4As % of Total RAM= 12.50
- 5Recommended kernel.shmmax (bytes)= 536,870,912
- 6Recommended kernel.shmall (pages)= 524,288
- 7sysctl Command= sysctl -w kernel.shmmax=536870912 kernel.shmall=524288
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why did PostgreSQL fail to start after I increased shared_buffers?
The most common cause is the requested segment exceeding kernel.shmmax, or the cumulative shared memory exceeding kernel.shmall — both must be raised via sysctl (and typically /etc/sysctl.conf for persistence) before increasing an application's shared memory configuration beyond the current limits.
What's the difference between System V shared memory and POSIX shared memory?
System V shm (shmget/shmat) is the older API governed by kernel.shmmax/shmall/shmmni; POSIX shared memory (shm_open, typically backed by tmpfs under /dev/shm) is governed instead by the size of the /dev/shm mount. Modern applications increasingly prefer POSIX shm, but many established databases still use System V shm.
How do I check current System V shared memory usage?
`ipcs -m` lists active shared memory segments with their sizes and attaching processes, and `ipcs -lm` shows the current kernel limits (shmmax, shmall, shmmni) for comparison against what your applications need.