Re: [PATCH v2 00/28] The new cgroup slab memory controller

[Roman Gushchin]

On Thu, Jan 30, 2020 at 07:36:26AM +0530, Bharata B Rao wrote:
> On Mon, Jan 27, 2020 at 09:34:25AM -0800, Roman Gushchin wrote:
> > The existing cgroup slab memory controller is based on the idea of
> > replicating slab allocator internals for each memory cgroup.
> > This approach promises a low memory overhead (one pointer per page),
> > and isn't adding too much code on hot allocation and release paths.
> > But is has a very serious flaw: it leads to a low slab utilization.
> > 
> > Using a drgn* script I've got an estimation of slab utilization on
> > a number of machines running different production workloads. In most
> > cases it was between 45% and 65%, and the best number I've seen was
> > around 85%. Turning kmem accounting off brings it to high 90s. Also
> > it brings back 30-50% of slab memory. It means that the real price
> > of the existing slab memory controller is way bigger than a pointer
> > per page.
> > 
> > The real reason why the existing design leads to a low slab utilization
> > is simple: slab pages are used exclusively by one memory cgroup.
> > If there are only few allocations of certain size made by a cgroup,
> > or if some active objects (e.g. dentries) are left after the cgroup is
> > deleted, or the cgroup contains a single-threaded application which is
> > barely allocating any kernel objects, but does it every time on a new CPU:
> > in all these cases the resulting slab utilization is very low.
> > If kmem accounting is off, the kernel is able to use free space
> > on slab pages for other allocations.
> > 
> > Arguably it wasn't an issue back to days when the kmem controller was
> > introduced and was an opt-in feature, which had to be turned on
> > individually for each memory cgroup. But now it's turned on by default
> > on both cgroup v1 and v2. And modern systemd-based systems tend to
> > create a large number of cgroups.
> > 
> > This patchset provides a new implementation of the slab memory controller,
> > which aims to reach a much better slab utilization by sharing slab pages
> > between multiple memory cgroups. Below is the short description of the new
> > design (more details in commit messages).
> > 
> > Accounting is performed per-object instead of per-page. Slab-related
> > vmstat counters are converted to bytes. Charging is performed on page-basis,
> > with rounding up and remembering leftovers.
> > 
> > Memcg ownership data is stored in a per-slab-page vector: for each slab page
> > a vector of corresponding size is allocated. To keep slab memory reparenting
> > working, instead of saving a pointer to the memory cgroup directly an
> > intermediate object is used. It's simply a pointer to a memcg (which can be
> > easily changed to the parent) with a built-in reference counter. This scheme
> > allows to reparent all allocated objects without walking them over and
> > changing memcg pointer to the parent.
> > 
> > Instead of creating an individual set of kmem_caches for each memory cgroup,
> > two global sets are used: the root set for non-accounted and root-cgroup
> > allocations and the second set for all other allocations. This allows to
> > simplify the lifetime management of individual kmem_caches: they are
> > destroyed with root counterparts. It allows to remove a good amount of code
> > and make things generally simpler.
> > 
> > The patchset* has been tested on a number of different workloads in our
> > production. In all cases it saved significant amount of memory, measured
> > from high hundreds of MBs to single GBs per host. On average, the size
> > of slab memory has been reduced by 35-45%.
> 
> Here are some numbers from multiple runs of sysbench and kernel compilation
> with this patchset on a 10 core POWER8 host:
> 
> ==========================================================================
> Peak usage of memory.kmem.usage_in_bytes, memory.usage_in_bytes and
> meminfo:Slab for Sysbench oltp_read_write with mysqld running as part
> of a mem cgroup (Sampling every 5s)
> --------------------------------------------------------------------------
> 				5.5.0-rc7-mm1	+slab patch	%reduction
> --------------------------------------------------------------------------
> memory.kmem.usage_in_bytes	15859712	4456448		72
> memory.usage_in_bytes		337510400	335806464	.5
> Slab: (kB)			814336		607296		25
> 
> memory.kmem.usage_in_bytes	16187392	4653056		71
> memory.usage_in_bytes		318832640	300154880	5
> Slab: (kB)			789888		559744		29
> --------------------------------------------------------------------------
> 
> 
> Peak usage of memory.kmem.usage_in_bytes, memory.usage_in_bytes and
> meminfo:Slab for kernel compilation (make -s -j64) Compilation was
> done from bash that is in a memory cgroup. (Sampling every 5s)
> --------------------------------------------------------------------------
> 				5.5.0-rc7-mm1	+slab patch	%reduction
> --------------------------------------------------------------------------
> memory.kmem.usage_in_bytes	338493440	231931904	31
> memory.usage_in_bytes		7368015872	6275923968	15
> Slab: (kB)			1139072		785408		31
> 
> memory.kmem.usage_in_bytes	341835776	236453888	30
> memory.usage_in_bytes		6540427264	6072893440	7
> Slab: (kB)			1074304		761280		29
> 
> memory.kmem.usage_in_bytes	340525056	233570304	31
> memory.usage_in_bytes		6406209536	6177357824	3
> Slab: (kB)			1244288		739712		40
> --------------------------------------------------------------------------
> 
> Slab consumption right after boot
> --------------------------------------------------------------------------
> 				5.5.0-rc7-mm1	+slab patch	%reduction
> --------------------------------------------------------------------------
> Slab: (kB)			821888		583424		29
> ==========================================================================
> 
> Summary:
> 
> With sysbench and kernel compilation,  memory.kmem.usage_in_bytes shows
> around 70% and 30% reduction consistently.
> 
> Didn't see consistent reduction of memory.usage_in_bytes with sysbench and
> kernel compilation.
> 
> Slab usage (from /proc/meminfo) shows consistent 30% reduction and the
> same is seen right after boot too.

That's just perfect!

memory.usage_in_bytes was most likely the same because the freed space
was taken by pagecache.

Thank you very much for testing!

Roman