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Re: Ideal Swap Partition Size

On Tue, 2009-01-20 at 20:06 -0800, Gordon Messmer wrote:
> Rick Stevens wrote:
> > 
> > Reserving a swap area and its size is rather dependent on what the
> > machine is doing.  We have database servers that, on occasion, get
> > hammered and revert to using swap for a brief time.  We use a 2X swap
> > size and we've come close to using it all, so it's still valid.  You
> > will have to watch it---as soon as you start really whacking swap,
> > system performance is going to start suffering quite badly.
> It's also important to bear in mind that under the standard 
> configuration, you must have at least as much "free" memory as the 
> largest application in your server, or else that application won't be 
> able to call external programs.

If you don't have enough memory (RAM+Swap) for the largest app you need
to run, then that app won't be able to run. And if you don't have enough
for the worst case combination of apps, then that combination won't be
able to run. It has nothing to do with the largest app calling external
programs or not.

> Let's imagine that you have a server with 2GB of RAM, and just 512MB of 
> swap (maybe based on the idea that swapping will cause the system to 
> behave badly).  Let's also imagine that you've tuned your SQL server to 
> keep as much data in memory as possible, so it's 1.5GB resident.  Now, 
> if you SQL server has helper applications that it wants to call, it has 
> to fork() and then exec() to start them.  When it does a fork(), the 
> system doesn't actually copy all of its pages for the new process, but 
> it does require that the memory be available (the extent to which that 
> is true depends on your overcommit settings).

This was true in older systems (actually the system just allocated space
for data and stack, since the code segment was shared) but Linux uses a
copy-on-write policy so I don't think it's true any more.

Of course the process can change its requirements at runtime, so it
might start running and then fail later, not because it's executing an
external program but simply through trying to allocate memory that the
system doesn't have.

>  However, since you don't 
> have 1.5GB of memory available, the fork() will probably fail, and the 
> SQL server process can't execute its helper script.

I don't think so (see above).

> This situation would be much harder to diagnose if you had 1GB of swap 
> and your SQL server were something like 1.3 GB.  In that case, it might 
> sometimes work and sometimes fail depending on how many other processes 
> were using memory.

And on how much the SQL process is using for a specific run.

> So, even if you expect to never *use* swap space, you should have at 
> least as much swap as physical RAM.

There is no "reasonable" amount of swap that will stop you from running
out of memory in *every* conceivable circumstance. You need to know the
behaviour of your system to make an educated guess.


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