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

From: Taras Glek
Date: Tue Apr 06 2010 - 22:55:14 EST

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:
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.
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

Suse ships their glibc with a dynamic linker patch to fadvise()
dynamic libraries(not sure why they switched from doing madvise
This is interesting. I wonder how SuSE implements the policy.
Do you have the patch or some strace output that demonstrates the
fadvise() call? in

As I recall they just fadvise the filedescriptor before accessing it.
I filed a glibc bug about this at . 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 :)


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