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Date:   Tue, 1 Mar 2022 07:18:39 +0100
From:   Oscar Salvador <osalvador@suse.de>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Feng Tang <feng.tang@intel.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Michal Hocko <mhocko@suse.com>,
        Rik van Riel <riel@surriel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Yang Shi <shy828301@gmail.com>, Zi Yan <ziy@nvidia.com>,
        Wei Xu <weixugc@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        zhongjiang-ali <zhongjiang-ali@linux.alibaba.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH -V13 2/3] NUMA balancing: optimize page placement for
 memory tiering system
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On Tue, Mar 01, 2022 at 09:16:18AM +0800, Huang, Ying wrote:
> Hi, Oscar,

Hi Huang Ying

>  b. Make kswapd of the fast memory node to reclaim pages until the free
>     pages are a little more than the high watermark (named as promo
>     watermark).  If we want to promote some hot pages from the slow
>     memory to the fast memory, but the free pages of the fast memory
>     node will become lower than the high watermark after promotion, we
>     will wake up kswapd of the fast memory node to demote more cold
>     pages in the fast memory node to the slow memory node firstly.  This
>     will free some extra space in the fast memory node, so the hot pages
>     in the slow memory node can be promoted to the fast memory node.

What about this? Somehow it sounds clear to me.

"b. Define a new watermark called wmark_promo which is higher than wmark_high,
    and have kswapd reclaiming pages until free pages reach such watermark.
    The scenario is as follows: when we want to promote hot-pages from a slow
    memory to a fast memory, but fast memory's free pages would go lower than
    high watermark with such promotion, we wake up kswapd with wmark_promo
    watermark in order to demote cold pages and free us up some space.
    So, next time we want to promote hot-pages we might have a chance of
    doing so."

But I am fine with your updated write up as well, so if you spin a new
version:

Reviewed-by: Oscar Salvador <osalvador@suse.de>


-- 
Oscar Salvador
SUSE Labs