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zsmalloc: document new fullness grouping
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Patch series "zsmalloc: minor documentation updates".

Two minor patches that bring zsmalloc documentation up to date.


This patch (of 2):

Update documentation and reflect new zspages fullness grouping (we don't
use almost_empty and almost_full anymore).

Link: https://lkml.kernel.org/r/20230325024631.2817153-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20230325024631.2817153-2-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Fixes: 67e157e ("zsmalloc: show per fullness group class stats")
Cc: Minchan Kim <minchan@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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@sergey-senozhatsky @akpm00
sergey-senozhatsky authored and akpm00 committed Apr 6, 2023
1 parent f76b3a3 commit 119b57e
Showing 1 changed file with 74 additions and 59 deletions.
133 changes: 74 additions & 59 deletions Documentation/mm/zsmalloc.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -39,13 +39,12 @@ With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via

# cat /sys/kernel/debug/zsmalloc/zram0/classes

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage
class size 10% 20% 30% 40% 50% 60% 70% 80% 90% 99% 100% obj_allocated obj_used pages_used pages_per_zspage freeable
...
...
9 176 0 1 186 129 8 4
10 192 1 0 2880 2872 135 3
11 208 0 1 819 795 42 2
12 224 0 1 219 159 12 4
30 512 0 12 4 1 0 1 0 0 1 0 414 3464 3346 433 1 14
31 528 2 7 2 2 1 0 1 0 0 2 117 4154 3793 536 4 44
32 544 6 3 4 1 2 1 0 0 0 1 260 4170 3965 556 2 26
...
...

Expand All @@ -54,10 +53,28 @@ class
index
size
object size zspage stores
almost_empty
the number of ZS_ALMOST_EMPTY zspages(see below)
almost_full
the number of ZS_ALMOST_FULL zspages(see below)
10%
the number of zspages with usage ratio less than 10% (see below)
20%
the number of zspages with usage ratio between 10% and 20%
30%
the number of zspages with usage ratio between 20% and 30%
40%
the number of zspages with usage ratio between 30% and 40%
50%
the number of zspages with usage ratio between 40% and 50%
60%
the number of zspages with usage ratio between 50% and 60%
70%
the number of zspages with usage ratio between 60% and 70%
80%
the number of zspages with usage ratio between 70% and 80%
90%
the number of zspages with usage ratio between 80% and 90%
99%
the number of zspages with usage ratio between 90% and 99%
100%
the number of zspages with usage ratio 100%
obj_allocated
the number of objects allocated
obj_used
Expand All @@ -67,18 +84,11 @@ pages_used
pages_per_zspage
the number of 0-order pages to make a zspage

We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where

* n = number of allocated objects
* N = total number of objects zspage can store
* f = fullness_threshold_frac(ie, 4 at the moment)

Similarly, we assign zspage to:

* ZS_ALMOST_FULL when n > N / f
* ZS_EMPTY when n == 0
* ZS_FULL when n == N

Each zspage maintains inuse counter which keeps track of the number of
objects stored in the zspage. The inuse counter determines the zspage's
"fullness group" which is calculated as the ratio of the "inuse" objects to
the total number of objects the zspage can hold (objs_per_zspage). The
closer the inuse counter is to objs_per_zspage, the better.

Internals
=========
Expand All @@ -94,10 +104,10 @@ of objects that each zspage can store.

For instance, consider the following size classes:::

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable
...
94 1536 0 0 0 0 0 3 0
100 1632 0 0 0 0 0 2 0
94 1536 0 .... 0 0 0 0 3 0
100 1632 0 .... 0 0 0 0 2 0
...


Expand Down Expand Up @@ -134,10 +144,11 @@ reduces memory wastage.

Let's take a closer look at the bottom of `/sys/kernel/debug/zsmalloc/zramX/classes`:::

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable

...
202 3264 0 0 0 0 0 4 0
254 4096 0 0 0 0 0 1 0
202 3264 0 .. 0 0 0 0 4 0
254 4096 0 .. 0 0 0 0 1 0
...

Size class #202 stores objects of size 3264 bytes and has a maximum of 4 pages
Expand All @@ -151,40 +162,42 @@ efficient storage of large objects.

For zspage chain size of 8, huge class watermark becomes 3632 bytes:::

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable

...
202 3264 0 0 0 0 0 4 0
211 3408 0 0 0 0 0 5 0
217 3504 0 0 0 0 0 6 0
222 3584 0 0 0 0 0 7 0
225 3632 0 0 0 0 0 8 0
254 4096 0 0 0 0 0 1 0
202 3264 0 .. 0 0 0 0 4 0
211 3408 0 .. 0 0 0 0 5 0
217 3504 0 .. 0 0 0 0 6 0
222 3584 0 .. 0 0 0 0 7 0
225 3632 0 .. 0 0 0 0 8 0
254 4096 0 .. 0 0 0 0 1 0
...

For zspage chain size of 16, huge class watermark becomes 3840 bytes:::

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable

...
202 3264 0 0 0 0 0 4 0
206 3328 0 0 0 0 0 13 0
207 3344 0 0 0 0 0 9 0
208 3360 0 0 0 0 0 14 0
211 3408 0 0 0 0 0 5 0
212 3424 0 0 0 0 0 16 0
214 3456 0 0 0 0 0 11 0
217 3504 0 0 0 0 0 6 0
219 3536 0 0 0 0 0 13 0
222 3584 0 0 0 0 0 7 0
223 3600 0 0 0 0 0 15 0
225 3632 0 0 0 0 0 8 0
228 3680 0 0 0 0 0 9 0
230 3712 0 0 0 0 0 10 0
232 3744 0 0 0 0 0 11 0
234 3776 0 0 0 0 0 12 0
235 3792 0 0 0 0 0 13 0
236 3808 0 0 0 0 0 14 0
238 3840 0 0 0 0 0 15 0
254 4096 0 0 0 0 0 1 0
202 3264 0 .. 0 0 0 0 4 0
206 3328 0 .. 0 0 0 0 13 0
207 3344 0 .. 0 0 0 0 9 0
208 3360 0 .. 0 0 0 0 14 0
211 3408 0 .. 0 0 0 0 5 0
212 3424 0 .. 0 0 0 0 16 0
214 3456 0 .. 0 0 0 0 11 0
217 3504 0 .. 0 0 0 0 6 0
219 3536 0 .. 0 0 0 0 13 0
222 3584 0 .. 0 0 0 0 7 0
223 3600 0 .. 0 0 0 0 15 0
225 3632 0 .. 0 0 0 0 8 0
228 3680 0 .. 0 0 0 0 9 0
230 3712 0 .. 0 0 0 0 10 0
232 3744 0 .. 0 0 0 0 11 0
234 3776 0 .. 0 0 0 0 12 0
235 3792 0 .. 0 0 0 0 13 0
236 3808 0 .. 0 0 0 0 14 0
238 3840 0 .. 0 0 0 0 15 0
254 4096 0 .. 0 0 0 0 1 0
...

Overall the combined zspage chain size effect on zsmalloc pool configuration:::
Expand Down Expand Up @@ -214,9 +227,10 @@ zram as a build artifacts storage (Linux kernel compilation).

zsmalloc classes stats:::

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable

...
Total 13 51 413836 412973 159955 3
Total 13 .. 51 413836 412973 159955 3

zram mm_stat:::

Expand All @@ -227,9 +241,10 @@ zram as a build artifacts storage (Linux kernel compilation).

zsmalloc classes stats:::

class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage freeable
class size 10% .... 100% obj_allocated obj_used pages_used pages_per_zspage freeable

...
Total 18 87 414852 412978 156666 0
Total 18 .. 87 414852 412978 156666 0

zram mm_stat:::

Expand Down

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