DTrace for Linux
With BrandZ, it's now possible to use DTrace on Linux applications. For the uninitiated, DTrace is the dynamic tracing facility in OpenSolaris; it allows for systemic analysis of a scope and precision unequalled in the industry. With DTrace, administrators and developers can trace low level services like I/O and scheduling, up the system stack through kernel functions calls, system calls, and system library calls, and into applications written in C and C++ or any of a host of dynamic languages like Java, Ruby, Perl or php. One of my contributions to BrandZ was to extend DTrace support for Linux binaries executed in a branded Zone.
DTrace has several different instrumentation providers that know how to instrument a particular part of the system and provide relevant probes for that component. The io provider lets you trace disk I/O, the fbt (function boundary tracing) provider lets you trace any kernel function call, etc. A typical system will start with more than 30,000 probes but providers can create probes dynamically to trace new kernel modules or user-land processes. When strictly focused on a user-land application, the most useful providers are typically the syscall provider to examine system calls and the pid provider that can trace any instruction in a any process executing on the system.
For Linux processes, the pid provider just worked (well, once Russ built a library to understand the Linux run-time linker), and we introduced a new provider -- the lx-syscall provider -- to trace entry and return for emulated Linux system calls. With these providers it's possible to understand every facet of a Linux application's behavior and with the other DTrace probes it's possible to reason about an application's use of system resources. In other words, you can take that sluggish Linux application, stick it in a branded Zone, dissect it using Solaris tools, and then bring it back to a native Linux system with the fruits of your DTrace investigation[1].
To give an example of using DTrace on Linux applications, I needed an application to examine. I wanted a well known program that either didn't run on Solaris or operated sufficiently differently such examining the Linux version rather than the Solaris port made sense. I decided on /usr/bin/top partly because of the dramatic differences between how it operates on Linux vs. Solaris (due to the differences in /proc), but mostly because of what I've heard my colleague, Bryan, refer to as the "top problem": your system is slow, so you run top. What's the top process? Top!
Running top in the Linux branded zone, I opened a shell in the global (Solaris) zone to use DTrace. I started as I do on Solaris applications: I looked at system calls. I was interested to see which system calls were being executed most frequently which is easily expressed in DTrace:
bash-3.00# dtrace -n lx-syscall:::entry'/execname == "top"/{ @[probefunc] = count(); }'
dtrace: description 'lx-syscall:::entry' matched 272 probes
^C
fstat64 322
access 323
gettimeofday 323
gtime 323
llseek 323
mmap2 323
munmap 323
select 323
getdents64 1289
lseek 1291
stat64 3545
rt_sigaction 5805
write 6459
fcntl64 6772
alarm 8708
close 11282
open 14827
read 14830
Note the use of the aggregation denoted with the '@'. Aggregations are the mechanism by which DTrace allows users to examine patterns of system behavior rather than examining each individual datum -- each system call for example. (In case you also noticed the strange discrepancy between the number of open and close system calls, many of those opens are failing so it makes sense that they would have no corresponding close. I used the lx-syscall provider to suss this out, but I omitted that investigation in a vain appeal for brevity.)
There may well be something fishy about this output, but nothing struck me as so compellingly fishy to explore immediately. Instead, I fired up vi and wrote a short D script to see which system calls were taking the most time:
lx-sys.d
#!/usr/sbin/dtrace -s
lx-syscall:::entry
/execname == "top"/
{
self->ts = vtimestamp;
}
lx-syscall:::return
/self->ts/
{
@[probefunc] = sum(vtimestamp - self->ts);
self->ts = 0;
}
This script creates a table of system calls and the time spent in them (in nanoseconds). The results were fairly interesting.
bash-3.00# ./lx-sys.d
dtrace: script './lx-sys.d' matched 550 probes
^C
llseek 4940978
gtime 5993454
gettimeofday 6603844
fstat64 14217312
select 26594875
lseek 30956093
mmap2 43463946
access 49033498
alarm 72216971
fcntl64 188281402
rt_sigaction 197646176
stat64 268188885
close 417574118
getdents64 781844851
open 1314209040
read 1862007391
write 2030173630
munmap 2195846497
That seems like a lot of time spent in munmap for top. In fact, I'm rather surprised that there's any mapping and unmapping going on at all (I guess that should have raised an eyebrow after my initial system call count). Unmapping memory is a pretty expensive operation that gets more expensive on bigger systems as it requires the kernel to do some work on every CPU to completely wipe out the mapping.
I then modified lx-sys.d to record the total amount of time top spent on the CPU and the total amount of time spent in system calls to see how large a chunk of time these seemingly expensive unmap operations were taking:
lx-sys2.d
#!/usr/sbin/dtrace -s
lx-syscall:::entry
/execname == "top"/
{
self->ts = vtimestamp;
}
lx-syscall:::return
/self->ts/
{
@[probefunc] = sum(vtimestamp - self->ts);
@["- all syscalls -"] = sum(vtimestamp - self->ts);
self->ts = 0;
}
sched:::on-cpu
/execname == "top"/
{
self->on = timestamp;
}
sched:::off-cpu
/self->on/
{
@["- total -"] = sum(timestamp - self->on);
self->on = 0;
}
I used the sched provider to see when top was going on and off of the CPU, and I added a row to record the total time spent in all system call. Here were the results (keep in mind I was just hitting ^C to end the experiment after a few seconds so it's expected that these numbers would be different from those above; there are ways to have more accurately timed experiments):
bash-3.00# ./lx-sys2.d
dtrace: script './lx-sys2.d' matched 550 probes
^C
llseek 939771
gtime 1088745
gettimeofday 1090020
fstat64 2494614
select 4566569
lseek 5186943
mmap2 7300830
access 8587484
alarm 11671436
fcntl64 31147636
rt_sigaction 33207341
stat64 45223200
close 69338595
getdents64 131196732
open 220188139
read 309764996
write 340413183
munmap 365830103
- all syscalls - 1589236337
- total - 3258101690
So system calls are consuming roughly half of top's time on the CPU and the munmap syscall is consuming roughly a quarter of that. This was enough to convince me that there was probably room for improvement and further investigation might bear fruit.
Next, I wanted to understand what this mapped memory was being used for so I wrote a little script that traces all the functions called in the process between when memory is mapped using the mmap2(2) system call and when it's unmapped and returned to the system through the munmap(2) system call:
map.d
#!/usr/sbin/dtrace -s
#pragma D option quiet
lx-syscall::mmap2:return
/pid == $target/
{
self->ptr = arg1;
self->depth = 10;
printf("%*.s depth, "", probefunc);
}
pid$target:::entry
/self->ptr/
{
self->depth++;
printf("%*.s -> %s`%s\n", self->depth, "", probemod, probefunc);
}
pid$target:::return
/self->ptr/
{
printf("%*.s depth, "", probemod, probefunc);
self->depth--;
}
lx-syscall::munmap:entry
/arg0 == self->ptr/
{
self->depth++;
printf("%*.s -> %s syscall\n", self->depth, "", probefunc);
self->ptr = 0;
self->depth = 0;
exit(0);
}
This script uses the $target variable which means that we need to run it with the -p option where is the process ID of top. When mmap2 returns, we set a thread local variable, 'ptr', which stores the address at the base of the mapped region; for every function entry and return in the process we call printf() if self->ptr is set; finally, we exit DTrace when munmap is called with that same address. Here are the results:
bash-3.00# ./map.d -p `pgrep top`
<- mmap2 syscall
<- LM2`libc.so.1`syscall
<- LM2`lx_brand.so.1`lx_emulate
<- LM2`lx_brand.so.1`lx_handler
<- libc.so.6`mmap
libc.so.6`memset
libc.so.6`cfree
libc.so.6`munmap
LM2`lx_brand.so.1`lx_handler
-> LM2`lx_brand.so.1`lx_emulate
-> LM2`libc.so.1`syscall
-> munmap syscall
I traced the probemod (shared object name) in addition to probefunc (function name) so that we'd be able to differentiate proper Linux functions (in this case all in libc.so.6) from functions in the emulation library (LM2`lx_brand.so.1). What we can glean from this is that the mmap call is a result of a call to malloc() and the unmap is due to a call to free(). What's unfortunate is that we're not seeing any function calls in top itself. For some reason, the top developer chose to strip this binary (presumably to save precious 2k the symbol table would have used on disk) so we're stuck with no symbolic information for top's functions and no ability to trace the individual function calls[2], but we can still reason about this a bit more.
A little analysis in mdb revealed that cfree (an alias for free) makes a tail-call to a function that calls munmap. It seems strange to me that freeing memory would immediately results in an unmap operation (since it would cause exactly the high overhead we're seeing here. To explore this, I wrote another script which looks at what proportion of calls to malloc result in a call to mmap():
malloc.d
#!/usr/sbin/dtrace -s
pid$target::malloc:entry
{
self->follow = arg0;
}
lx-syscall::mmap2:entry
/self->follow/
{
@["mapped"] = count();
self->follow = 0;
}
pid$target::malloc:return
/self->follow/
{
@["no map"] = count();
self->follow = 0;
}
Here are the results:
bash-3.00# ./malloc.d -p `pgrep top`
dtrace: script './malloc.d' matched 11 probes
^C
mapped 275
no map 3024
So a bunch of allocations result in a mmap, but not a huge number. Next I decided to explore if there might be a correlation between the size of the allocation and whether or not it resulted in a call to mmap using the following script:
malloc2.d
#!/usr/sbin/dtrace -s
pid$target::malloc:entry
{
self->size = arg0;
}
lx-syscall::mmap2:entry
/self->size/
{
@["mapped"] = quantize(self->size);
self->size = 0;
}
pid$target::malloc:return
/self->size/
{
@["no map"] = quantize(self->size);
self->size = 0;
}
Rather than just counting the frequency, I used the quantize aggregating action to built a power-of-two histogram on the number of bytes being allocated (self->size). The output was quite illustrative:
bash-3.00# ./malloc2.d -p `pgrep top`
dtrace: script './malloc2.d' matched 11 probes
^C
no map
value ------------- Distribution ------------- count
2 | 0
4 |@@@@@@@ 426
8 |@@@@@@@@@@@@@@@ 852
16 |@@@@@@@@@@@ 639
32 |@@@@ 213
64 | 0
128 | 0
256 | 0
512 |@@@@ 213
1024 | 0
mapped
value ------------- Distribution ------------- count
131072 | 0
262144 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 213
524288 | 0
All the allocations that required a mmap were huge -- between 256k and 512k. Now it makes sense why the Linux libc allocator would treat these allocations a little differently than reasonably sized allocations. And this is clearly a smoking gun for top performance: it would do much better to preallocate a huge buffer and grow it as needed (assuming it actually needs it at all) than to malloc it each time. Tracking down the offending line of code would just require a non-stripped binary and a little DTrace invocation like this:
# dtrace -n pid`pgrep top`::malloc:entry'/arg0 >= 262144/{@[ustack()] = count()}'
From symptoms to root cause on a Linux application in a few DTrace scripts -- and it took me approximately 1000 times longer to cobble together some vaguely coherent prose describing the scripts than it did for me to actually perform the investigation. BrandZ opens up some pretty interesting new vistas for DTrace. I look forward to seeing Linux applications being brought in for tune-ups on BrandZ and then reaping those benefits either back on their mother Linux or sticking around to enjoy the fault management, ZFS, scalability, and, of course, continued access to DTrace in BrandZ.
[1] Of course, results may vary since the guts of the Linux kernel differ significantly from those of the Solaris kernel, but they're often fairly similar. I/O or scheduling problems will be slightly different, but often not so different that the conclusions lack applicability. [2] Actually, we can can still trace function calls -- in fact, we can trace any instruction -- but it takes something of a heroic effort. We could disassemble parts of top to identify calls sites and then use esoteric pid123::-:_address_ probe format to trace the stripped function. I said we could do it; I never said it would be pretty.
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