From: Matthew Wilcox <willy@infradead.org>
To: Bernd Schubert <bernd.schubert@fastmail.fm>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
Miklos Szeredi <miklos@szeredi.hu>,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org
Subject: Re: sequential 1MB mmap read ends in 1 page sync read-ahead
Date: Fri, 14 Apr 2023 23:02:32 +0100 [thread overview]
Message-ID: <ZDnNeKt1bPWb2PzC@casper.infradead.org> (raw)
In-Reply-To: <c59e54f9-6eae-3c35-bce8-ac03af84b3ed@fastmail.fm>
On Fri, Apr 14, 2023 at 10:47:39PM +0200, Bernd Schubert wrote:
> > Up to bs=512K it works fine, 1M (and for what it matters
> > already 768K) introduce the order=0 issue.
>
> Hmm, I replaced memcpy with dumb version, that copies byte by byte - problem
> gone. Is it possible that the optimized memcpy causes caused kind of random
> memory access and confuses mm / readahead?
> And somehow your memcpy or system is not doing that?
Oh, that would make sense! If the memcpy() works high-to-low, then
you'd see exactly the behaviour you're reporting. Whereas low-to-high
results in the behaviour I'm seeing.
Hm. I don't know what to do about that. Maybe a "sufficiently large"
memcpy should call posix_madvise(src, n, POSIX_MADV_WILLNEED)
next prev parent reply other threads:[~2023-04-14 22:02 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <aae918da-833f-7ec5-ac8a-115d66d80d0e@fastmail.fm>
2023-04-13 21:33 ` Bernd Schubert
2023-04-14 4:07 ` Matthew Wilcox
2023-04-14 15:05 ` Bernd Schubert
2023-04-14 19:52 ` Bernd Schubert
2023-04-14 20:47 ` Bernd Schubert
2023-04-14 22:02 ` Matthew Wilcox [this message]
2023-04-17 15:10 ` Bernd Schubert
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