On 03.04.23 18:34, Stefan Roesch wrote:
In contrast to e.g.:
1) THP resulted in many zeropages we end up deduplicating again. The THP
placement was unfortunate.
2) Unoptimized memory allocators that leave many identical pages mapped
after freeing up memory (e.g., zeroed pages, pages all filled with
poison values) instead of e.g., using MADV_DONTNEED to free up that
memory.
I repeated an experiment with and without KSM. In terms of THP there is
no huge difference between the two. On a 64GB main memory machine I see
between 100 - 400MB in AnonHugePages.
/sys/kernel/mm/ksm/pages_shared is over 10000 when we run this on an
Instagram workload. The workload consists of 36 processes plus a few
sidecar processes.
Thanks! To which value is /sys/kernel/mm/ksm/max_page_sharing set in that
environment?
It's set to the standard value of 256.
In the meantime I have run experiments with different settings for
pages_to_scan. With the default value of 100, we only get a relatively
small benefit of KSM. If I increase the value to for instance to 2000 or
3000 the savings are substantial. (The workload is memory bound, not
CPU bound).
Interesting.
Here are some stats for setting pages_to_scan to 3000:
full_scans: 560
general_profit: 20620539008
max_page_sharing: 256
merge_across_nodes: 1
pages_shared: 125446
pages_sharing: 5259506
pages_to_scan: 3000
pages_unshared: 1897537
pages_volatile: 12389223
run: 1
sleep_millisecs: 20
stable_node_chains: 176
stable_node_chains_prune_millisecs: 2000
stable_node_dups: 2604
use_zero_pages: 0
zero_pages_sharing: 0
What would be interesting is pages_shared after max_page_sharing was set to a
very high number such that pages_shared does not include duplicates. Then
pages_shared actually expresses how many different pages we deduplicate. No need
to run without THP in that case.
Thats on my list for the next set of experiments.
Splendid.
Similarly, enabling "use_zero_pages" could highlight if your workload ends up
deduplciating a lot of zeropages. But maxing out max_page_sharing would be
sufficient to understand what's happening.
I already run experiments with use_zero_pages, but they didn't make a
difference. I'll repeat the experiment with a higher pages_to_scan
value.
Okay, so it's most certainly not the zeropage. Thanks for that
information and running the experiments!
--
Thanks,
David / dhildenb
