Re: Downsides to madvise/fadvise(willneed) for application startup

[Taras Glek]

On 04/07/2010 12:38 AM, Wu Fengguang wrote:
> On Wed, Apr 07, 2010 at 10:54:58AM +0800, Taras Glek 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.
> >      
> Boot time user space readahead can do better than kernel heuristic
> readahead in several ways:
>
> - it can collect better knowledge on which files/pages will be used
>    which lead to high readahead hit ratio and less cache consumption
>
> - it can submit readahead requests for many files in parallel,
>    which enables queuing (elevator, NCQ etc.) optimizations
>
> So I won't call it dirty hack :)
>
>    
Fair enough.
> > > 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
> >      
> Sure. It will do read-around from current fault offset - 64kb to +64kb.
>    
That's excellent.
>    
> > dynamic linker read binary sections backwards causes faults.
> > http://sourceware.org/bugzilla/show_bug.cgi?id=11447
> >      
> There are too many data in
> http://people.mozilla.com/~tglek/startup/systemtap_graphs/ld_bug/report.txt
> Can you show me the relevant lines? (wondering if I can ever find such lines..)
>    
The first part of the file lists sections in a file and their hex 
offset+size.

lines like 0 512 offset(#1) mean a read at position 0 of 512 bytes. 
Incidentally this first read is coming from vfs_read, so the log doesn't 
take account readahead (unlike the other reads caused by mmap page faults).

So
15310848 131072 offset(#2)=====================
eaa73c 1523c .bss
eaa73c 19d1e .comment

15142912 131072 offset(#3)=====================
e810d4 200 .dynamic
e812d4 470 .got
e81744 3b50 .got.plt
e852a0 2549c .data

Shows 2 reads where the dynamic linker first seeks to the end of the 
file(to zero out .bss, causing IO via COW) and the backtracks to
read in .dynamic. However you are right, all of the backtracking reads 
are over 64K.
Thanks for explaining that. I am guessing your change to boost 
readaround will fix this issue nicely for firefox.

> > >        
> > > > > 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:
> >      
> 550 Can't open
> /pub/linux/distributions/suse/pub/suse/update/10.1/rpm/src/glibc-2.4-31.12.3.src.rpm:
> No such file or directory
>
> OK I give up.
>
>    
> > As I recall they just fadvise the filedescriptor before accessing it.
> >      
> Obviously this is a bit risky for small memory systems..
>
>    
> > > > > 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.
> >      
> Right.
>
>    
> > 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.
> >      
> Agreed.
>
>    
> > 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.
> >      
> That's great. When will we enjoy your research fruits? :)
>    
Released it yesterday. Hopefully other bloated binaries will benefit 
from this too.

http://blog.mozilla.com/tglek/2010/04/07/icegrind-valgrind-plugin-for-optimizing-cold-startup/

Thanks a lot Wu, I feel I understand the kernel side of what's happening 
now.

Taras
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