On Thu, Sep 03, 2015 at 09:09:24AM +0300, Pekka Enberg wrote: > Hi Andrew, > > On Wed, 2 Sep 2015 22:10:12 -0500 (CDT) Christoph Lameter <cl@xxxxxxxxx> wrote: > >> > But I'd still like some pointers/help on what makes slab merging so > >> > beneficial. I'm sure Christoph and others have justification. But if > >> > not then yes the default to slab merging probably should be revisited. > >> > >> ... > >> > >> Check out the linux-mm archives for these dissussions. > > On Thu, Sep 3, 2015 at 7:55 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > Somewhat OT, but... The question Mike asks should be comprehensively > > answered right there in the switch-to-merging patch's changelog. > > > > The fact that it is not answered in the appropriate place and that > > we're reduced to vaguely waving at the list archives is a fail. And a > > lesson! > > Slab merging is a technique to reduce memory footprint and memory > fragmentation. Joonsoo reports 3% slab memory reduction after boot > when he added the feature to SLAB: I'm not sure whether you are trying to indicate that it was justified inteh commit message or indicate how little justification there was... > commit 12220dea07f1ac6ac717707104773d771c3f3077 > Author: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> > Date: Thu Oct 9 15:26:24 2014 -0700 > > mm/slab: support slab merge > > Slab merge is good feature to reduce fragmentation. If new creating slab > have similar size and property with exsitent slab, this feature reuse it > rather than creating new one. As a result, objects are packed into fewer > slabs so that fragmentation is reduced. A partial page or two in a newly allocated slab in not "fragmentation". They are simply free objects in the cache that haven't been allocated yet. Fragmentation occurs when large numbers of objects are freed so the pages end up mostly empty but cannot be freed because there is still 1 or 2 objects in use of them. As such, if there was fragementation and slab merging fixed it, I'd expect to be seeing a much larger reduction in memory usage.... > Below is result of my testing. > > * After boot, sleep 20; cat /proc/meminfo | grep Slab > > <Before> > Slab: 25136 kB > > <After> > Slab: 24364 kB > > We can save 3% memory used by slab. The numbers don't support the conclusion. Memory used from boot to boot always varies by a small amount - a slight difference in the number of files accessed by the boot process can account for this. Also, you can't 't measure slab fragmentation by measuring the amount of memory used. You have to look at object counts in each slab and work out the percentage of free vs allocated objects. So there's no evidence that this 772kb difference in memory footprint can even be attributed to slab merging. What about the rest of the slab fragmentation problem space? It's not even mentioned in the commit, but that's really what is important to long running machines. IOWs, where's description of the problem that needs fixing? What's the example workload that demonstrates the problem? What's the before and after measurements of the workloads that generate significant slab fragmentation? What's the long term impact of the change (e.g. a busy server with a uptime of several weeks)? is the fragmentation level reduced? increased? not significant? What impact does this have on subsystems with shrinkers that are now operating on shared slabs? Do the shrinkers still work as effectively as they used to? Do they now cause slab fragmentation, and if they do, does it self correct under continued memory pressure? And with the patch being merged without a single reviewed-by or acked-by, I'm sitting here wondering how we managed to fail software engineering 101 so badly here? Cheers, Dave. -- Dave Chinner dchinner@xxxxxxxxxx -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
