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

[Taras Glek <tglek@mozilla.com>]

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.

>    
>>> Also, once an application is started is it reasonable to keep it
>>> madvise(WILLNEED)ed or should the madvise flags be reset?
>>>        
>> It's a one-time operation that starts immediate readahead, no permanent
>> changes are done.
>>      
> Right. The kernel regard WILLNEED as a readahead request from userspace.
>
>    
>>> Perhaps the kernel could monitor the page-in patterns to increase the
>>> readahead sizes? This may already happen, I've noticed that a handful of
>>> pagefaults trigger>  131072bytes of IO, perhaps this just needs tweaking.
>>>        
>> CCd the man :-)
>>      
> Thank you :)
>
> Cheers,
> Fengguang
>    

Cheers,
Taras

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