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SubjectRe: Higher slub memory consumption on 64K page-size systems?
On Wed, Oct 28, 2020 at 05:07:57PM -0700, Roman Gushchin wrote:
> On Wed, Oct 28, 2020 at 11:20:30AM +0530, Bharata B Rao wrote:
> > I have mostly looked at reducing the slab memory consumption here.
> > But I do understand that default tunable values have been arrived
> > at based on some benchmark numbers. Are there ways or possibilities
> > to reduce the slub memory consumption with the existing level of
> > performance is what I would like to understand and explore.
>
> Hi Bharata!
>
> I wonder how the distribution of the consumed memory by slab_caches
> differs between 4k and 64k pages. In particular, I wonder if
> page-sized and larger kmallocs make the difference (or a big part of it)?
> There are many places in the kernel which are doing something like
> kmalloc(PAGE_SIZE).

Here is comparision of topmost slabs in terms of memory usage b/n
4K and 64K configurations:

Case 1: After boot
==================
4K page-size
------------
Name Objects Objsize Space Slabs/Part/Cpu O/S O %Fr %Ef Flg
inode_cache 23382 592 14.1M 400/0/33 54 3 0 97 a
dentry 29484 192 5.7M 592/0/110 42 1 0 98 a
kmalloc-1k 5358 1024 5.6M 130/9/42 32 3 5 97
task_struct 371 9856 4.1M 88/6/40 3 3 4 87
kmalloc-512 6640 512 3.4M 159/3/49 32 2 1 99
...
kmalloc-4k 530 4096 2.2M 42/6/27 8 3 8 96

64K page-size
-------------
pgtable-2^11 935 16384 38.7M 16/16/58 16 3 21 39
inode_cache 23980 592 14.4M 203/0/17 109 0 0 98 a
thread_stack 709 16384 12.0M 6/1/17 32 3 4 96
task_struct 1012 9856 10.4M 4/1/16 53 3 5 95
kmalloc-64k 144 65536 9.4M 2/0/16 8 3 0 100

Case 2: After hackbench run
===========================
4K page-size
------------
inode_cache 21823 592 13.3M 361/3/46 54 3 0 96 a
kmalloc-512 10309 512 9.4M 433/325/146 32 2 56 55
kmalloc-1k 6207 1024 6.5M 121/12/78 32 3 6 97
dentry 28923 192 5.9M 468/48/261 42 1 6 92 a
task_struct 418 9856 5.1M 106/24/51 3 3 15 80
...
kmalloc-4k 510 4096 2.1M 41/10/26 8 3 14 95

64K page-size
-------------
kmalloc-8k 3081 8192 84.9M 241/241/83 32 2 74 29
thread_stack 2919 16384 52.4M 15/10/85 32 3 10 91
pgtable-2^11 1281 16384 50.8M 20/20/77 16 3 20 41
task_struct 3771 9856 40.3M 9/6/68 53 3 7 92
vm_area_struct 92295 200 18.9M 8/8/281 327 0 2 97
...
kmalloc-64k 144 65536 9.4M 2/0/16 8 3 0 100

I can't see any specific pattern wrt to kmalloc cache usage in both the
above cases (boot vs hackbench run). In the boot case, the 64K configuration
consuming more memory can be attributed probably to the bigger page size
itself. However in case of hackbench run, any significant number of
partial slabs does contribute to significant increase of memory for
64K configuration.

>
> Re slub tuning: in general we do care about the number of objects
> in a partial list, less about the number of pages. If we can have the
> same amount of objects but on fewer pages, it's even better.

Right, but how do we achieve that when few number of inuse objects are
spread across a number of partial slabs? This specifically is the case
we see after a workload run (hackbench in this case)

> So I don't see any reasons why we shouldn't scale down these tunables
> if the PAGE_SIZE > 4K.
> Idk if it makes sense to switch to byte-sized tunables or just to hardcode
> custom default values for the 64k page case. The latter is probably
> is easier.

Right, tuning the mininum number of objects when calculating the page order
of the slab and tuning cpu_partial value show some consistent reduction
in the slab memory consumption. (I have shown this in previous mail)

Thanks for your comments.

Regards,
Bharata.

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