linux-kernel - Re: [PATCH 3/4] selftests: memcg: Adjust expected reclaim values of protected cgroups

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Message-ID: <Yn6pBPq+lAXm9NG8@carbon>
Date:   Fri, 13 May 2022 11:52:52 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     void@...ifault.com, akpm@...ux-foundation.org,
        cgroups@...r.kernel.org, hannes@...xchg.org, kernel-team@...com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        mhocko@...nel.org, shakeelb@...gle.com, tj@...nel.org,
        Richard Palethorpe <rpalethorpe@...e.de>
Subject: Re: [PATCH 3/4] selftests: memcg: Adjust expected reclaim values of
 protected cgroups

On Fri, May 13, 2022 at 07:18:10PM +0200, Michal Koutny wrote:
> The numbers are not easy to derive in a closed form (certainly mere
> protections ratios do not apply), therefore use a simulation to obtain
> expected numbers.
> 
> The new values make the protection tests succeed more precisely.
> 
> 	% run as: octave-cli script
> 	%
> 	% Input configurations
> 	% -------------------
> 	% E parent effective protection
> 	% n nominal protection of siblings set at the givel level
> 	% c current consumption -,,-
> 
> 	% example from testcase (values in GB)
> 	E = 50 / 1024;
> 	n = [75 25 0 500 ] / 1024;
> 	c = [50 50 50 0] / 1024;
> 
> 	% Reclaim parameters
> 	% ------------------
> 
> 	% Minimal reclaim amount (GB)
> 	cluster = 32*4 / 2**20;
> 
> 	% Reclaim coefficient (think as 0.5^sc->priority)
> 	alpha = .1
> 
> 	% Simulation parameters
> 	% ---------------------
> 	epsilon = 1e-7;
> 	timeout = 1000;
> 
> 	% Simulation loop
> 	% ---------------------
> 	% Simulation assumes siblings consumed the initial amount of memory (w/out
> 	% reclaim) and then the reclaim starts, all memory is reclaimable, i.e. treated
> 	% same. It simulates only non-low reclaim and assumes all memory.min = 0.
> 
> 	ch = [];
> 	eh = [];
> 	rh = [];
> 
> 	for t = 1:timeout
> 		% low_usage
> 		u = min(c, n);
> 		siblings = sum(u);
> 
> 		% effective_protection()
> 		protected = min(n, c);                % start with nominal
> 		e = protected * min(1, E / siblings); % normalize overcommit
> 
> 		% recursive protection
> 		unclaimed = max(0, E - siblings);
> 		parent_overuse = sum(c) - siblings;
> 		if (unclaimed > 0 && parent_overuse > 0)
> 			overuse = max(0, c - protected);
> 			e += unclaimed * (overuse / parent_overuse);
> 		endif
> 
> 		% get_scan_count()
> 		r = alpha * c;             % assume all memory is in a single LRU list
> 
> 		% commit 1bc63fb1272b ("mm, memcg: make scan aggression always exclude protection")
> 		sz = max(e, c);
> 		r .*= (1 - (e+epsilon) ./ (sz+epsilon));
> 
> 		% uncomment to debug prints
> 		% e, c, r
> 
> 		% nothing to reclaim, reached equilibrium
> 		if max(r) < epsilon
> 			break;
> 		endif
> 
> 		% SWAP_CLUSTER_MAX
> 		r = max(r, (r > epsilon) .* cluster);
> 		% XXX here I do parallel reclaim of all siblings
> 		% in reality reclaim is serialized and each sibling recalculates own residual
> 		c = max(c - r, 0);
> 
> 		ch = [ch ; c];
> 		eh = [eh ; e];
> 		rh = [rh ; r];
> 	endfor
> 
> 	t
> 	c, e

This is a cool stuff!

How about to place it into a separate file and add a comment into the code
with a reference?

Thanks!