linux-kernel - Re: [PATCH v6 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order

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Message-ID: <Y32aZCYlAzONnaC4@google.com>
Date:   Wed, 23 Nov 2022 12:58:28 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, minchan@...nel.org,
        ngupta@...are.org, senozhatsky@...omium.org, sjenning@...hat.com,
        ddstreet@...e.org, vitaly.wool@...sulko.com
Subject: Re: [PATCH v6 4/6] zsmalloc: Add a LRU to zs_pool to keep track of
 zspages in LRU order

On (22/11/18 16:15), Nhat Pham wrote:
> +#ifdef CONFIG_ZPOOL
> +	/* Move the zspage to front of pool's LRU */
> +	if (mm == ZS_MM_WO) {
> +		if (!list_empty(&zspage->lru))
> +			list_del(&zspage->lru);
> +		list_add(&zspage->lru, &pool->lru);
> +	}
> +#endif

Just an idea.

Have you considered having size class LRU instead of pool LRU?

Evicting pages from different classes can have different impact on the
system, in theory. For instance, ZS_FULL zspage in class size 3264
(bytes) holds 5 compressed objects per-zspage, which are 5 compressed
swapped out pages. While zspage in a class size 176 (bytes) holds 93
compressed objects (swapped pages). Both zspages consist of 4
non-contiguous 0-order physical pages, so when we free zspage from these
classes we release 4 physical pages. However, in terms of major
page faults evicting a page from size class 3264 looks better than from
a size class 176: 5 major page faults vs 93 major page faults.

Does this make sense?