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

[Bharata B Rao]

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.

Regards,
Bharata.