Windows – How many users can be logged in at the same time


How many Windows users can be logged in at the same time on a local computer? I.e. how many sessions can be opened at the same time? Is there a limitation?

More detailed:

I can use "tasklist.exe" to see the tasks running on my computer. If I run "tasklist.exe" as Administrator, it shows all the tasks, not only for the current user. Session IDs will be different: 0 for System, 1 for the first logged user, 2 for the second one.

So the question(s) is(are) how many users can run their tasks on a local computer at the same time? How many sessions (with different session IDs) can exist on a local computer at the same time? Is there a limitation?

Best Answer

The short answer is: apart from arbitrary limitations of your Windows edition, as many as you can until you exhaust system resources (usually available memory).

Sysinternals' Mark Russinovich wrote some time ago a blog series called Pushing the Limits of Window in which he explore the different limits that Windows could face in the different parts of the operating system. Except a couple of hard-coded values most of the limits came from system architecture (32/64 bits) or available memory, either physical RAM or RAM + pagefile. He talks about the structure of sessions in the six part and don't mention any specific limit (note that the four desktop limit he mentions is for Sysinternals desktop, not the system limit).

Since I didn't have a server edition at hand I made a little test in a VM with Windows 7 Enterprise (32 bits) with 1.5GB of RAM. This works because the limit for client editions of Windows is one interactive session, you can open as many sessions as you want but if you activate another session (for example using Remote Desktop) the current interactive session is locked. Those are the results:

One and two opened sessions (only desktop)

One and two sessions only desktop

Six opened sessions (only desktop)

Six sessions only desktop

As expected the more sessions opened the more memory is used, in this case about 50 MB per session, which is not much, but this is a barebone virtual machine that doesn't load any programs at startup nor have bloated drivers.

Of course this changes as you start opening programs, I opened Firefox in each session with one or two tabs each (nothing too heavy, SuperUser in the main session and the Firefox welcome pages in the other ones):

Six opened sessions (Firefox with a one/two of tabs in each session)

Six sessions with Firefox

As you can see the memory usage almost doubled, and this is just with one application opened, in a real system you would add all the programs that are opened at session start (all those icons in the notification area plus others without windows nor icons) and all the programs that you are going to use.

So in practice, unless you have a buggy program that leaks the objects with hard-coded values mentioned earlier I think that the main limit is physical memory. Although available memory include the pagefile before you reach the system limit you would reach the usable limit: as memory gets full Windows move more things to the pagefile and the file cache is almost nonexistent so the hard disk begin to be trashed (in the captures you can see that with 6 sessions and no apps the file cache is 560MB but with Firefox opened it was reduced to 87MB).

As an example of this my girlfriend worked some time ago in a public office where they used thin clients and logged in a Windows server using Remote Desktop or Citrix (I'm not sure), with people doing most of the work in the remote sessions. The problem was that they have more users than the ones the system could support without problems so just opening session could take 5-7 minutes easily.

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