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.

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?

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.

Taras Glek

PS. For more details on this issue see my blog at
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