Re: [dm-devel] [PATCH] dm: Make MIN_IOS, et al, tunable via sysctl.

[Frank Mayhar <fmayhar@google.com>]

On Tue, 2013-08-20 at 17:22 -0400, Mikulas Patocka wrote:
> On Fri, 16 Aug 2013, Frank Mayhar wrote:
> > The device mapper and some of its modules allocate memory pools at
> > various points when setting up a device.  In some cases, these pools are
> > fairly large, for example the multipath module allocates a 256-entry
> > pool and the dm itself allocates three of that size.  In a
> > memory-constrained environment where we're creating a lot of these
> > devices, the memory use can quickly become significant.  Unfortunately,
> > there's currently no way to change the size of the pools other than by
> > changing a constant and rebuilding the kernel.
> I think it would be better to set the values to some low value (1 should 
> be enough, there is 16 in some places that is already low enough). There 
> is no need to make it user-configurable, because the user will see no 
> additional benefit from bigger mempools.
> 
> Maybe multipath is the exception - but other dm targets don't really need 
> big mempools so there is no need to make the size configurable.

The point is not to make the mempools bigger, the point is to be able to
make them _smaller_ for memory-constrained environments.  In some cases,
even 16 can be too big, especially when creating a large number of
devices.

In any event, it seems unfortunate to me that these values are
hard-coded.  One shouldn't have to rebuild the kernel to change them,
IMHO.
-- 
Frank Mayhar
310-460-4042