Downsides to madvise/fadvise(willneed) for application startup
From: Taras Glek
Date: Mon Apr 05 2010 - 19:06:19 EST
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.
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.
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
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?
Also, once an application is started is it reasonable to keep it
madvise(WILLNEED)ed or should the madvise flags be reset?
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.
PS. For more details on this issue see my blog at
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