Re: [ATTEND] many topics

From: NeilBrown <neilb@suse.com>
To: Theodore Ts'o <tytso@mit.edu>, Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>,
	lsf-pc@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org
Subject: Re: [ATTEND] many topics
Date: Tue, 24 Jan 2017 06:34:29 +1100	[thread overview]
Message-ID: <87mvehd0ze.fsf@notabene.neil.brown.name> (raw)
In-Reply-To: <20170123170924.ubx2honzxe7g34on@thunk.org>

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On Tue, Jan 24 2017, Theodore Ts'o wrote:

> On Sun, Jan 22, 2017 at 10:05:44PM -0800, Matthew Wilcox wrote:
>> 
>> I don't have a clear picture in my mind of when Java promotes objects
>> from nursery to tenure
>
> It's typically on the order of minutes.   :-)
>
>> ... which is not too different from my lack of
>> understanding of what the MM layer considers "temporary" :-)  Is it
>> acceptable usage to allocate a SCSI command (guaranteed to be freed
>> within 30 seconds) from the temporary area?  Or should it only be used
>> for allocations where the thread of control is not going to sleep between
>> allocation and freeing?
>
> What the mm folks have said is that it's to prevent fragmentation.  If
> that's the optimization, whether or not you the process is allocating
> the memory sleeps for a few hundred milliseconds, or even seconds, is
> really in the noise compared with the average lifetime of an inode in
> the inode cache, or a page in the page cache....
>
> Why do you think it matters whether or not we sleep?  I've not heard
> any explanation for the assumption for why this might be important.

Because "TEMPORARY" implies a limit to the amount of time, and sleeping
is the thing that causes a process to take a large amount of time.  It
seems like an obvious connection to me.

Imagine I want to allocate a large contiguous region in the
ZONE_MOVEABLE region.  I find a mostly free region, so I just need to
move those last few pages.  If there is a limit on how long a process
can sleep while holding an allocation from ZONE_MOVEABLE, then I know
how long, at most, I need to wait before those pages become either free
or movable.  If those processes can wait indefinitely, then I might have
to wait indefinitely to get this large region.

"temporary" doesn't mean anything without a well defined time limit.

But maybe I completely misunderstand.

NeilBrown

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