Upstream: pending
This basically removed workingset_activation and reduced calls to
workingset_age_nonresident.
The idea behind this change is a new way to calculate the refault
distance and prepare for adapting refault distance based file page
protection for multi-gen LRU.
Currently, refault distance re-activation for active/inactive can help
keep working set pages in memory, it works by estimating the refault
(re-access) distance of a page, if it's small enough, then put it
on active LRU instead of inactive LRU.
The estimation, as described in mm/workingset.c, is based on two assumptions:
1. Activation of an inactive page will left-shift LRU pages (considering
LRU starts from right).
2. Eviction of an inactive page will left-shift LRU pages.
Assumption 2 is correct, but assumption 1 is not always true, an activated
page could be anywhere in the LRU list (through mark_page_accessed), it
only left-shift the pages on its right side.
And besides, one page can get activate/deactivated for multiple times.
And multi-gen LRU doesn't fit with this model well, pages are getting
aged in generations, and getting promoted frequently between generations.
So instead we introduce a simpler idea here: Just presume the evicted
pages are still in memory, each has an corresponding eviction timestamp
(nonresistence_age) that is increased and recorded upon each eviction.
These timestamp could logically form a "Shadow LRU", a read-only
imaginary LRU. Let the `nonresistence_age` still be NA, then we have:
Let SP = ((NA's reading @ current) - (NA's reading @ eviction))
+-memory available to cache-+
| |
+-------------------------+===============+===========+
| * shadows O O O | INACTIVE | ACTIVE |
+-+-----------------------+===============+===========+
| |
+-----------------------+
| SP
fault page O -> Hole left by refaulted in pages.
Entries are suppose to be removed
upon access but this is not a real
LRU so can't really update it.
* -> The page corresponding to SP
It can be easily seen that SP stands for the offset of a page in the
imaginary LRU, which is also how far the current workflow could push
a page out of available memory. Since all evicted page was once head
of INACTIVE list, the estimated minimum value of refault distance is:
SP + NR_INACTIVE
On refault, the page *may* get activated and stay in memory if we put
it to active LRU if:
SP + NR_INACTIVE < NR_INACTIVE + NR_ACTIVE
Which can be simplified to:
SP < NR_ACTIVE
Then the page is worth getting re-activated to start from active LRU,
since the access distance is smaller than the total memory.
And since this is only an estimation, based on several hypotheses, and
it could break the ability of LRU to distinguish a workingset out of
caches, in extreme cases all refault causing activation will lead to
worse thrashing, so throttle this by two factors:
1. Notice previously re-faulted in pages may leave "holes" on the shadow
part of LRU, that part is left unhandled on purpose to decrease
re-activate rate for pages that have a large SP value (the larger
SP value a page has, the more likely it will be affected by such
holes).
2. When the active LRU is long enough, chanllaging active pages
by re-activating a one-time access previously evicted/inactive page
may not be a good idea, so throttle the re-activation when
NR_ACTIVE > NR_INACTIVE, by comparing with NR_INACTIVE instead.
Another effect of the refault activation throttling worth noticing is that,
when the cache size is larger than total memory and hotness is similar
among all cache pages, it can help hold a portion (possible have slightly
higher hotness) of the caches in memory instead of letting caches get
evicted permutably due to the nature of LRU.
That's because the established workingset (active LRU) will tend to stay
since we throttled reactivation when NR_ACTIVE is high.
This side effect is actually similar with the algoritm before, which
introduce such effect by increasing nonresistence_age in extra call
paths, trottled the re-activation when activition/reactivation is
massively happenning.
Combined all above, we have following simple rules:
Upon refault, if any of following conditions is met, mark page as active:
- If active LRU is low (NR_ACTIVE < NR_INACTIVE), check if:
SP < NR_ACTIVE
- If active LRU is high (NR_ACTIVE >= NR_INACTIVE), check if:
SP < NR_INACTIVE
Code-wise, this is simpler than before since no longer need to do lruvec
workingset data update when activating a page, and so far, a few benchmarks
shows a similar or better result under memore pressure. The performance
should also be better when there is no memory pressure since some memcg
iteration and atomic operation is no longer needed.
When combined with multi-gen LRU (in later commits) it shows a measurable
performance gain for some workloads.
Using memtier and fio test from commit ac35a49023 but scaled down
to fit in my test environment, and some other test results:
memtier test (with 16G ramdisk as swap and 4G memcg limit on an i7-9700):
memcached -u nobody -m 16384 -s /tmp/memcached.socket \
-a 0766 -t 12 -B binary &
memtier_benchmark -S /tmp/memcached.socket -P memcache_binary -n allkeys\
--key-minimum=1 --key-maximum=32000000 --key-pattern=P:P -c 1 \
-t 12 --ratio 1:0 --pipeline 8 -d 2000 -x 6
fio test 1 (with 16G ramdisk on 28G VM on an i7-9700):
fio -name=refault --numjobs=12 --directory=/mnt --size=1024m \
--buffered=1 --ioengine=io_uring --iodepth=128 \
--iodepth_batch_submit=32 --iodepth_batch_complete=32 \
--rw=randread --random_distribution=random --norandommap \
--time_based --ramp_time=5m --runtime=5m --group_reporting
fio test 2 (with 16G ramdisk on 28G VM on an i7-9700):
fio -name=mglru --numjobs=10 --directory=/mnt --size=1536m \
--buffered=1 --ioengine=io_uring --iodepth=128 \
--iodepth_batch_submit=32 --iodepth_batch_complete=32 \
--rw=randread --random_distribution=zipf:1.2 --norandommap \
--time_based --ramp_time=10m --runtime=5m --group_reporting
mysql (using oltp_read_only from sysbench, with 12G of buffer pool
in a 10G memcg):
sysbench /usr/share/sysbench/oltp_read_only.lua <auth and db params> \
--tables=36 --table-size=2000000 --threads=12 --time=1800
kernel build test done with 3G memcg limit on an i7-9700.
Before (Average of 6 test run):
fio: IOPS=5125.5k
fio2: IOPS=7291.16k
memcached: 57600.926 ops/s
mysql: 6280.08 tps
kernel-build: 1817.13499 seconds
After (Average of 6 test run):
fio: IOPS=5137.5k (+2.3%)
fio2: IOPS=7300.67k (+1.3%)
memcached: 57878.422 ops/s (+4.8%)
mysql: 6312.06 tps (+0.5%)
kernel-build: 1813.66231 seconds (+2.0%)
Signed-off-by: Kairui Song <kasong@tencent.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.
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