Re: [LSF/MM/BPF TOPIC] Memory fragmentation with large block sizes

["Theodore Tso" <tytso@mit.edu>]

On Thu, Feb 19, 2026 at 10:54:48AM +0100, Hannes Reinecke wrote:
> Hi all,
> 
> I (together with the Czech Technical University) did some experiments trying
> to measure memory fragmentation with large block sizes.
> Testbed used was an nvme setup talking to a nvmet storage over
> the network.
> 
> Doing so raised some challenges:
> 
> - How do you _generate_ memory fragmentation? The MM subsystem is
>   precisely geared up to avoid it, so you would need to come up
>   with some idea how to defeat it. With the help from Willy I managed
>   to come up with something, but I really would like to discuss
>   what would be the best option here.

I'm trying to understand the goal of the experiment.  I'm guessing
that the goal was to see how much memory fragmentation would result
from using large block sizes with the control being to use, say, 4k
blocks.  Is that correct?

So I guess the question here is what are realstic workloads that
people would have in real world situations, so we can do the A-B
experiments to see what using LBS result in?

> - What is acceptable memory fragmentation? Are we good enough if the
>   measured fragmentation does not grow during the test runs?

I can think of two possible metrics.  The first is whether it results
in degradation of performance given certain real world workloads.

The second is whether given a particular memory pressure, the memory
fragmentation results in more jobs getting OOM killed.

	      	      	      	   	   - Ted