Here is the scenario:
1) Build image with some app X and httpserver API module.
2) Start app X using REST API /app call with new_program=false (same
namespace)
3) OSv loads app library and any dependent ones from disk and executes it.
4) Start app X again same way as in 2) over and over again (unlimited
times) BUT OSv skips load any libraries from disk instead re-uses relevant
ELF objects already loaded/mmaped in memory.
This actually works using simple native-example. But fails on a second run
with ffmpeg app where I get this type of stack trace:
0x00000000003a9482 in processor::cli_hlt () at arch/x64/processor.hh:247
247 asm volatile ("cli; hlt" : : : "memory");
(gdb) bt
#0 0x00000000003a9482 in processor::cli_hlt () at arch/x64/processor.hh:247
#1 arch::halt_no_interrupts () at arch/x64/arch.hh:48
#2 osv::halt () at arch/x64/power.cc:24
#3 0x000000000023f144 in abort (fmt=fmt@entry=0x63335b "Aborted\n") at
runtime.cc:132
#4 0x0000000000202765 in abort () at runtime.cc:98
#5 0x000000000045dc9c in osv::generate_signal (siginfo=...,
ef=0xffff800003099068) at libc/signal.cc:124
#6 0x000000000045dd0b in osv::handle_mmap_fault (addr=<optimized out>,
sig=<optimized out>, ef=<optimized out>) at libc/signal.cc:139
#7 0x0000000000347bf3 in mmu::vm_fault (addr=0, addr@entry=56,
ef=ef@entry=0xffff800003099068) at core/mmu.cc:1338
#8 0x00000000003a32de in page_fault (ef=0xffff800003099068) at
arch/x64/mmu.cc:38
#9 <signal handler called>
#10 0x0000100003d39d6c in ?? ()
#11 0x00002000003ff220 in ?? ()
#12 0xffffa00002ce7ada in ?? ()
#13 0x0000000000000000 in ?? ()
If I set new_program=true ffmpeg runs fine every time up to 32 times.
So my motivation is to be able to run given application on OSv sequentially
one by one without having to restart OSv and only pay price for loading
code in memory once on the first run.
I have not analyzed OSv code well enough to understand if that is what
already happens. But as the ffmpeg example shows it comes with some quirks.
Also this would be best effort and only work if app cleans itself after
completion which I think is the case for ffmpeg but may not the case for
all kinds of apps.
Looking at elf.cc it looks like the elf segments get unmapped in
remove_objects() that is setup to be called once they are no longer
referenced. So ideally we would want to have an option of skipping
unmapping the ELF so that when app is re-executed again we skip the mmaping
part as code would be still in memory.
I do not understand ELF lifecycle well enough to know if that is even
possible.
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