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

From: Taras Glek
Date: Tue Apr 06 2010 - 17:57:46 EST

On 04/06/2010 02:51 AM, 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.

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).

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.
Define idle situations. Do you mean that madv(willneed) will aggresively readahead, but only while cpu(or disk?) is idle?
I am trying to optimize application startup which means that the cpu is busy while not blocked on io.
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.

Can't wait to hear the juicy details.
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.
I may be measuring this wrong, but in my experience the only change madvise(willneed) does in increase the length parameter to __do_page_cache_readahead(). My script is at .

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