linux-kernel - Re: [PATCH 8/8] mm: Remove __GFP

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Date:   Thu, 19 Oct 2017 15:18:14 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 8/8] mm: Remove __GFP_COLD

On 10/18/2017 09:59 AM, Mel Gorman wrote:
> As the page free path makes no distinction between cache hot and cold
> pages, there is no real useful ordering of pages in the free list that
> allocation requests can take advantage of. Juding from the users of
> __GFP_COLD, it is likely that a number of them are the result of copying
> other sites instead of actually measuring the impact. Remove the
> __GFP_COLD parameter which simplifies a number of paths in the page
> allocator.
> 
> This is potentially controversial but bear in mind that the size of the
> per-cpu pagelists versus modern cache sizes means that the whole per-cpu
> list can often fit in the L3 cache. Hence, there is only a potential benefit
> for microbenchmarks that alloc/free pages in a tight loop. It's even worse
> when THP is taken into account which has little or no chance of getting a
> cache-hot page as the per-cpu list is bypassed and the zeroing of multiple
> pages will thrash the cache anyway.
> 
> The truncate microbenchmarks are not shown as this patch affects the
> allocation path and not the free path. A page fault microbenchmark was
> tested but it showed no sigificant difference which is not surprising given
> that the __GFP_COLD branches are a miniscule percentage of the fault path.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

Acked-by: Vlastimil Babka <vbabka@...e.cz>