Performance analysis of Linux Kernel Markers 0.20 for 2.6.17

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

Hi,

Following the huge discussion thread about tracing/static vs dynamic
instrumentation/markers, a consensus seems to emerge about the need for a
marker system in the Linux kernel. The main issues this mechanism addresses are:

- Identify code important to runtime data collection/analysis tools in tree so
  that it follows the code changes naturally.
- Be visually appealing to kernel developers.
- Have a very low impact on the system performance.
- Integrate in the standard kernel infrastructure : use C and loadable modules.

The time has come for some performance measurements of the Linux Kernel Markers,
which follows. I attach a PDF with tables and charts which condense these
results.


* Micro-benchmarks

Use timestamp counter to calculate the time spent, with interrupts disabled.
Machine : Pentium 4 3GHz, 1GB ram
Fully preemptible kernel
Linux Kernel Markers 0.19
Kernel : Linux 2.6.17

marker : MARK(subsys_mark1, "%d %p", 1, NULL);

This marker, with two elements (integer and pointer) have been chosen because it
is representative of high volume events. For instance, a trap entry event logs a
trap_id (long) and an address (pointer). The same applies to system calls, where
a system call entry event logs both the ID of the system call and the address of
the caller.


* Execute an empty loop

- Without marker
NR_LOOPS : 10000000
time delta (cycles): 15026497
cycles per loop : 1.50

- i386 "optimized" : immediate value, test and predicted branch
  (non connected marker)
NR_LOOPS : 10000000
time delta (cycles): 40031640
cycles per loop : 4.00
cycles per loop for marker : 4.00-1.50=2.50

- i386 "generic" : load, test and predicted branch
  (non connected marker)
NR_LOOPS : 10000000
time delta (cycles): 26697878
cycles per loop : 2.67
cycles per loop for marker : 2.67-1.50=1.17


* Execute a loop of memcpy 4096 bytes

This test has been done to show the impact of markers on a system where the
memory is already used, which is more representative of a running kernel.

- Without marker
NR_LOOPS : 10000
time delta (cycles): 12981555
cycles per loop : 1298.16

- i386 "optimized" : immediate value, test and predicted branch
  (non connected marker)
NR_LOOPS : 10000
time delta (cycles): 12982290
cycles per loop : 1298.23
cycles per loop for marker : 1298.23-1298.16=0.074

- i386 "generic" : load, test and predicted branch
  (non connected marker)
NR_LOOPS : 10000
time delta (cycles): 13002788
cycles per loop : 1300.28
cycles per loop for marker : 1300.28-1298.16=2.123


The following tests are done with the "optimized" markers only

- Execute a loop with marker enabled, with i386 "fastcall" register argument
  setup, probe empty. With preemption disabling.
NR_LOOPS : 100000
time delta (cycles): 4407608
cycles per loop : 44.08
cycles per loop to disable preemption and setup arguments in registers :
44.08-4.00=40.08

- Execute a loop with a marker enabled, with an empty probe. Var args argument
  setup, probe empty. With preemption disabling.
NR_LOOPS : 100000
time delta (cycles): 5210587
cycles per loop : 52.11
additional cycles per loop to setup var args : 52.11-44.08=8.03

- Execute a loop with a marker enabled, with an empty probe. Var args argument
  setup, probe empty. No preemption disabling.
NR_LOOPS : 100000
time delta (cycles): 3363450
cycles per loop : 33.63
cycles per loop to disable preemption : 44.08-33.63=10.45

- Execute a loop with marker enabled, with i386 "asmlinkage" arguments expected.
  Data is copied by the probe. With preemption disabling.
NR_LOOPS : 100000
time delta (cycles): 5299837
cycles per loop : 53.00
additional cycles per loop to get arguments in probe (from stack) on x86 :
53.00-52.11=0.89

- Execute a loop with marker enabled, with var args probe expecting arguments.
  Data is copied by the probe. With preemption disabling.
NR_LOOPS : 100000
time delta (cycles): 5574300
cycles per loop : 55.74
additional cycles per loop to get expected variable arguments on x86 : 
  55.74-53.00=2.74

- Execute a loop with marker enabled, with var args probe, format string
  Data is copied by the probe. This is a 6 bytes string to decode.
NR_LOOPS : 100000
time delta (cycles): 9622117
cycles per loop : 96.22
additional cycles per loop to dynamically parse arguments with a 6 bytes format
string : 96.22-55.74=40.48

- Execute a loop with marker enabled, with var args probe expecting arguments.
  Data is copied by the probe. With preemption disabling. An empty "kprobe" is
  connected to the probe.
NR_LOOPS : 100000
time delta (cycles): 423397455
cycles per loop : 4233.97
additional cycles per loop to execute the kprobe : 4233.97-55.74=4178.23


* Assembly code

The disassembly of the following marked function will be shown :

static int my_open(struct inode *inode, struct file *file)
{
        MARK(subsys_mark1, "%d %p", 1, NULL);

        return -EPERM;
}


- Optimized

static int my_open(struct inode *inode, struct file *file)
{
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 0c                sub    $0xc,%esp
        MARK(subsys_mark1, "%d %p", 1, NULL);
   6:   b0 00                   mov    $0x0,%al <-- immediate load 0 in al
   8:   84 c0                   test   %al,%al
   a:   75 07                   jne    13 <my_open+0x13>

        return -EPERM;
}
   c:   b8 ff ff ff ff          mov    $0xffffffff,%eax
  11:   c9                      leave  
  12:   c3                      ret    
  13:   b8 01 00 00 00          mov    $0x1,%eax
  18:   e8 fc ff ff ff          call   19 <my_open+0x19> <-- preempt_disable
  1d:   c7 44 24 08 00 00 00    movl   $0x0,0x8(%esp)
  24:   00 
  25:   c7 44 24 04 01 00 00    movl   $0x1,0x4(%esp)
  2c:   00 
  2d:   c7 04 24 0d 00 00 00    movl   $0xd,(%esp)
  34:   ff 15 74 10 00 00       call   *0x1074 <-- function pointer
  3a:   b8 01 00 00 00          mov    $0x1,%eax
  3f:   e8 fc ff ff ff          call   40 <my_open+0x40> <-- preempt_enable
  44:   eb c6                   jmp    c <my_open+0xc>


- Generic 

static int my_open(struct inode *inode, struct file *file)
{
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 0c                sub    $0xc,%esp
        MARK(subsys_mark1, "%d %p", 1, NULL);
   6:   0f b6 05 20 10 00 00    movzbl 0x1020,%eax <-- memory load byte
   d:   84 c0                   test   %al,%al
   f:   75 07                   jne    18 <my_open+0x18>

        return -EPERM;
}
  11:   b8 ff ff ff ff          mov    $0xffffffff,%eax
  16:   c9                      leave  
  17:   c3                      ret    
  18:   b8 01 00 00 00          mov    $0x1,%eax
  1d:   e8 fc ff ff ff          call   1e <my_open+0x1e> <-- preempt_disable
  22:   c7 44 24 08 00 00 00    movl   $0x0,0x8(%esp)
  29:   00 
  2a:   c7 44 24 04 01 00 00    movl   $0x1,0x4(%esp)
  31:   00 
  32:   c7 04 24 0d 00 00 00    movl   $0xd,(%esp)
  39:   ff 15 74 10 00 00       call   *0x1074 <-- function pointer
  3f:   b8 01 00 00 00          mov    $0x1,%eax
  44:   e8 fc ff ff ff          call   45 <my_open+0x45> <-- preempt_enable
  49:   eb c6                   jmp    11 <my_open+0x11>



Here is the typical var arg probe that has been used in those tests. It saves
the values expectes as parameters in global variables. The DO_MARK1_FORMAT
define is used for probe registration to make sure that it will be connected
with a marker that has a matching format string. Note that this checking is
optional : the probe can register with a NULL format and afterward check itself
the format string received in parameter dynamically.

int value;
void *ptr;

#define DO_MARK1_FORMAT "%d %p"
void do_mark1(const char *format, ...)
{
        va_list ap;

        va_start(ap, format);
        value = va_arg(ap, int);
        ptr = va_arg(ap, void*);

        va_end(ap);
}


Here is the disassembly of the probe :

#define DO_MARK1_FORMAT "%d %p"
void do_mark1(const char *format, ...)
{
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 04                sub    $0x4,%esp
        va_list ap;

        va_start(ap, format);
        value = va_arg(ap, int);
   6:   8b 45 0c                mov    0xc(%ebp),%eax
   9:   a3 00 00 00 00          mov    %eax,0x0
        ptr = va_arg(ap, void*);
   e:   8b 45 10                mov    0x10(%ebp),%eax
  11:   a3 00 00 00 00          mov    %eax,0x0
        
        va_end(ap);
}
  16:   c9                      leave  
  17:   c3                      ret    


* Size (x86)

This is the size added by each marker to the memory image :

- Optimized

.text section : instructions
Adds 6 bytes in the "likely" path.
Adds 32 bytes in the "unlikely" path.
.data section : r/w data
0 byte
.rodata.str1 : strings
Length of the marker name
.debug_str : strings (if loaded..)
Length of the marker name + 7 bytes (__mark_)
.markers
8 bytes (2 pointers)
.markers.c
12 bytes (3 pointers)

- Generic

.text section : instructions
Adds 11 bytes in the "likely" path.
Adds 32 bytes in the "unlikely" path.
.data section : r/w data
1 byte (the activation flag)
.rodata.str1 : strings
Length of the marker name
.debug_str : strings (if loaded..)
Length of the marker name + 7 bytes (__mark_)
.markers
8 bytes (2 pointers)
.markers.c
12 bytes (3 pointers)


* Macro-benchmarks

Compiling a 2.6.17 kernel on a Pentium 4 3GHz, 1GB ram, cold cache.
Running a 2.6.17 vanilla kernel :
real    8m2.443s
user    7m35.124s
sys     0m34.950s

Running a 2.6.17 kernel with lttng-0.6.0pre11 markers (no probe connected) :
real    8m1.635s
user    7m34.552s
sys     0m36.298s

--> 0.98 % speedup with markers

Ping flood on loopback interface :
Running a 2.6.17 vanilla kernel :
136596 packets transmitted, 136596 packets received, 0% packet loss
round-trip min/avg/max = 0.0/0.0/0.1 ms

real    0m10.840s
user    0m0.360s
sys     0m10.485s

12601 packets transmitted/s

Running a 2.6.17 kernel with lttng-0.6.0pre11 markers (no probe connected) :
108504 packets transmitted, 108504 packets received, 0% packet loss
round-trip min/avg/max = 0.0/0.0/0.1 ms

real    0m8.614s
user    0m0.264s
sys     0m8.353s

12596 packets transmitted/s

--> 0.03 % slowdown with markers


Conclusion

In an empty loop, the generic marker is faster than the optimized marker. This
may be due to better performances of the movzbl instruction over the movb on the
Pentium 4 architecture. However, when we execute a loop of 4kB copy, the impact
of the movzbl becomes greater because it uses the memory bandwidth.

The preemption disabling and call to a probe itself costs 48.11 cycles, almost
as much as dynamically parsing the format string to get the variable arguments
(40.48 cycles).

There is almost no difference, on x86, between passing the arguments directly on
the stack and using a variable argument list when its layout is known
statically (0.89 cycles vs 2.74 cycles).

The int3 approach for adding instrumentation dynamically saves the 0.074 cycle
(typcal use, high memory usage) used by the optimized marker by adding the
ability to insert a breakpoint at any location without any impact on the code
when inactive. This breakpoint based approach is very useful to instrument core
kernel code that has not been previously marked without need to recompile and
reboot. We can therefore compare the case "without markers" to the null impact
of an inactive int3 breakpoint.

However, the performance impact for using a kprobe is non negligible when
activated. Assuming that kprobes would have a mechanism to get the variables
from the caller's stack, it would perform the same task in at least 4178.23
cycles vs 55.74 for a marker and a probe (ratio : 75). While kprobes are very
useful for the reason explained earlier, the high event rate paths in the kernel
would clearly benefit from a marker mechanism when the are probed.

Code size and memory footprints are smaller with the optimized version : 6
bytes of code in the likely path compared to 11 bytes. The memory footprint of
the optimized approach saves 4 bytes of data memory that would otherwise have to
stay in cache.

On the macro-benchmark side, no significant difference in performance has been
found between the vanilla kernel and a kernel "marked" with the standard LTTng
instrumentation.




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