Date:	Wed, 7 Apr 2010 15:14:08 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Taras Glek <tglek@...illa.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Downsides to madvise/fadvise(willneed) for application startup

On Wed, Apr 07, 2010 at 12:06:07PM +0800, Minchan Kim wrote:
> On Wed, Apr 7, 2010 at 11:54 AM, Taras Glek <tglek@...illa.com> wrote:
> > On 04/06/2010 07:24 PM, Wu Fengguang wrote:
> >>
> >> Hi Taras,
> >>
> >> On Tue, Apr 06, 2010 at 05:51:35PM +0800, Johannes Weiner wrote:
> >>
> >>>
> >>> On Mon, Apr 05, 2010 at 03:43:02PM -0700, Taras Glek wrote:
> >>>
> >>>>
> >>>> Hello,
> >>>> I am working on improving Mozilla startup times. It turns out that page
> >>>> faults(caused by lack of cooperation between user/kernelspace) are the
> >>>> main cause of slow startup. I need some insights from someone who
> >>>> understands linux vm behavior.
> >>>>
> >>
> >> How about improve Fedora (and other distros) to preload Mozilla (and
> >> other apps the user run at the previous boot) with fadvise() at boot
> >> time? This sounds like the most reasonable option.
> >>
> >
> > That's a slightly different usecase. I'd rather have all large apps startup
> > as efficiently as possible without any hacks. Though until we get there,
> > we'll be using all of the hacks we can.
> >>
> >> As for the kernel readahead, I have a patchset to increase default
> >> mmap read-around size from 128kb to 512kb (except for small memory
> >> systems).  This should help your case as well.
> >>
> >
> > Yes. Is the current readahead really doing read-around(ie does it read pages
> > before the one being faulted)? From what I've seen, having the dynamic
> > linker read binary sections backwards causes faults.
> > http://sourceware.org/bugzilla/show_bug.cgi?id=11447
> >>
> >>
> >>>>
> >>>> Current Situation:
> >>>> The dynamic linker mmap()s  executable and data sections of our
> >>>> executable but it doesn't call madvise().
> >>>> By default page faults trigger 131072byte reads. To make matters worse,
> >>>> the compile-time linker + gcc lay out code in a manner that does not
> >>>> correspond to how the resulting executable will be executed(ie the
> >>>> layout is basically random). This means that during startup 15-40mb
> >>>> binaries are read in basically random fashion. Even if one orders the
> >>>> binary optimally, throughput is still suboptimal due to the puny
> >>>> readahead.
> >>>>
> >>>> IO Hints:
> >>>> Fortunately when one specifies madvise(WILLNEED) pagefaults trigger 2mb
> >>>> reads and a binary that tends to take 110 page faults(ie program stops
> >>>> execution and waits for disk) can be reduced down to 6. This has the
> >>>> potential to double application startup of large apps without any clear
> >>>> downsides.
> >>>>
> >>>> Suse ships their glibc with a dynamic linker patch to fadvise()
> >>>> dynamic libraries(not sure why they switched from doing madvise
> >>>> before).
> >>>>
> >>
> >> This is interesting. I wonder how SuSE implements the policy.
> >> Do you have the patch or some strace output that demonstrates the
> >> fadvise() call?
> >>
> >
> > glibc-2.3.90-ld.so-madvise.diff in
> > http://www.rpmseek.com/rpm/glibc-2.4-31.12.3.src.html?hl=com&cba=0:G:0:3732595:0:15:0:
> >
> > As I recall they just fadvise the filedescriptor before accessing it.
> >>
> >>
> >>>>
> >>>> I filed a glibc bug about this at
> >>>> http://sourceware.org/bugzilla/show_bug.cgi?id=11431 . Uli commented
> >>>> with his concern about wasting memory resources. What is the impact of
> >>>> madvise(WILLNEED) or the fadvise equivalent on systems under memory
> >>>> pressure? Does the kernel simply start ignoring these hints?
> >>>>
> >>>
> >>> It will throttle based on memory pressure.  In idle situations it will
> >>> eat your file cache, however, to satisfy the request.
> >>>
> >>> Now, the file cache should be much bigger than the amount of unneeded
> >>> pages you prefault with the hint over the whole library, so I guess the
> >>> benefit of prefaulting the right pages outweighs the downside of evicting
> >>> some cache for unused library pages.
> >>>
> >>> Still, it's a workaround for deficits in the demand-paging/readahead
> >>> heuristics and thus a bit ugly, I feel.  Maybe Wu can help.
> >>>
> >>
> >> Program page faults are inherently random, so the straightforward
> >> solution would be to increase the mmap read-around size (for desktops
> >> with reasonable large memory), rather than to improve program layout
> >> or readahead heuristics :)
> >>
> >
> > Program page faults may exhibit random behavior once they've started.
> >
> > During startup page-in pattern of over-engineered OO applications is very
> > predictable. Programs are laid out based on compilation units, which have no
> > relation to how they are executed. Another problem is that any large old
> > application will have lots of code that is either rarely executed or
> > completely dead. Random sprinkling of live code among mostly unneeded code
> > is a problem.
> > I'm able to reduce startup pagefaults by 2.5x and mem usage by a few MB with
> > proper binary layout. Even if one lays out a program wrongly, the worst-case
> > pagein pattern will be pretty similar to what it is by default.
> >
> > But yes, I completely agree that it would be awesome to increase the
> > readahead size proportionally to available memory. It's a little silly to be
> > reading tens of megabytes in 128kb increments :)  You rock for trying to
> > modernize this.
> 
> Hi, Wu and Taras.
> 
> I have been watched at this thread.
> That's because I had a experience on reducing startup latency of application
> in embedded system.
> 
> I think sometime increasing of readahead size wouldn't good in embedded.
> Many of embedded system has nand as storage and compression file system.
> About nand, as you know, random read effect isn't rather big than hdd.
> About compression file system, as one has a big compression,
> it would make startup late(big block read and decompression).
> We had to disable readahead of code page with kernel hacking.
> And it would make application slow as time goes by.
> But at that time we thought latency is more important than performance
> on our application.
> 
> Of course, it is different whenever what is file system and
> compression ratio we use .
> So I think increasing of readahead size might always be not good.
> 
> Please, consider embedded system when you have a plan to tweak
> readahead, too. :)

Minchan, glad to know that you have experiences on embedded Linux.

While increasing the general readahead size from 128kb to 512kb, I
also added a limit for mmap read-around: if system memory size is less
than X MB, then limit read-around size to X KB. For example, do only
128KB read-around for a 128MB embedded box, and 32KB ra for 32MB box.

Do you think it a reasonable safety guard? Patch attached.

Thanks,
Fengguang


View attachment "readahead-small-memory-limit-readaround.patch" of type "text/x-diff" (1887 bytes)

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