Perf ring buffer¶
1. Introduction¶
The ring buffer is a fundamental mechanism for data transfer. perf uses ring buffers to transfer event data from kernel to user space, another kind of ring buffer which is so called auxiliary (AUX) ring buffer also plays an important role for hardware tracing with Intel PT, Arm CoreSight, etc.
The ring buffer implementation is critical but it’s also a very challenging work. On the one hand, the kernel and perf tool in the user space use the ring buffer to exchange data and stores data into data file, thus the ring buffer needs to transfer data with high throughput; on the other hand, the ring buffer management should avoid significant overload to distract profiling results.
This documentation dives into the details for perf ring buffer with two parts: firstly it explains the perf ring buffer implementation, then the second part discusses the AUX ring buffer mechanism.
2. Ring buffer implementation¶
2.1 Basic algorithm¶
That said, a typical ring buffer is managed by a head pointer and a tail pointer; the head pointer is manipulated by a writer and the tail pointer is updated by a reader respectively.
+---------------------------+
| | |***|***|***| | |
+---------------------------+
`-> Tail `-> Head
* : the data is filled by the writer.
Figure 1. Ring buffer
Perf uses the same way to manage its ring buffer. In the implementation there are two key data structures held together in a set of consecutive pages, the control structure and then the ring buffer itself. The page with the control structure in is known as the “user page”. Being held in continuous virtual addresses simplifies locating the ring buffer address, it is in the pages after the page with the user page.
The control structure is named as perf_event_mmap_page, it contains a
head pointer data_head and a tail pointer data_tail. When the
kernel starts to fill records into the ring buffer, it updates the head
pointer to reserve the memory so later it can safely store events into
the buffer. On the other side, when the user page is a writable mapping,
the perf tool has the permission to update the tail pointer after consuming
data from the ring buffer. Yet another case is for the user page’s
read-only mapping, which is to be addressed in the section
2.3.3 Writing samples into buffer.
user page ring buffer
+---------+---------+ +---------------------------------------+
|data_head|data_tail|...| | |***|***|***|***|***| | | |
+---------+---------+ +---------------------------------------+
` `----------------^ ^
`----------------------------------------------|
* : the data is filled by the writer.
Figure 2. Perf ring buffer
When using the perf record tool, we can specify the ring buffer size
with option -m or --mmap-pages=, the given size will be rounded up
to a power of two that is a multiple of a page size. Though the kernel
allocates at once for all memory pages, it’s deferred to map the pages
to VMA area until the perf tool accesses the buffer from the user space.
In other words, at the first time accesses the buffer’s page from user
space in the perf tool, a data abort exception for page fault is taken
and the kernel uses this occasion to map the page into process VMA
(see perf_mmap_fault()), thus the perf tool can continue to access
the page after returning from the exception.
2.2 Ring buffer for different tracing modes¶
The perf profiles programs with different modes: default mode, per thread mode, per cpu mode, and system wide mode. This section describes these modes and how the ring buffer meets requirements for them. At last we will review the race conditions caused by these modes.
2.2.1 Default mode¶
Usually we execute perf record command followed by a profiling program
name, like below command:
perf record test_program
This command doesn’t specify any options for CPU and thread modes, the perf tool applies the default mode on the perf event. It maps all the CPUs in the system and the profiled program’s PID on the perf event, and it enables inheritance mode on the event so that child tasks inherits the events. As a result, the perf event is attributed as:
evsel::cpus::map[] = { 0 .. _SC_NPROCESSORS_ONLN-1 }
evsel::threads::map[] = { pid }
evsel::attr::inherit = 1
These attributions finally will be reflected on the deployment of ring buffers. As shown below, the perf tool allocates individual ring buffer for each CPU, but it only enables events for the profiled program rather than for all threads in the system. The T1 thread represents the thread context of the ‘test_program’, whereas T2 and T3 are irrelevant threads in the system. The perf samples are exclusively collected for the T1 thread and stored in the ring buffer associated with the CPU on which the T1 thread is running.
T1 T2 T1
+----+ +-----------+ +----+
CPU0 |xxxx| |xxxxxxxxxxx| |xxxx|
+----+--------------+-----------+----------+----+-------->
| |
v v
+-----------------------------------------------------+
| Ring buffer 0 |
+-----------------------------------------------------+
T1
+-----+
CPU1 |xxxxx|
-----+-----+--------------------------------------------->
|
v
+-----------------------------------------------------+
| Ring buffer 1 |
+-----------------------------------------------------+
T1 T3
+----+ +-------+
CPU2 |xxxx| |xxxxxxx|
--------------------------+----+--------+-------+-------->
|
v
+-----------------------------------------------------+
| Ring buffer 2 |
+-----------------------------------------------------+
T1
+--------------+
CPU3 |xxxxxxxxxxxxxx|
-----------+--------------+------------------------------>
|
v
+-----------------------------------------------------+
| Ring buffer 3 |
+-----------------------------------------------------+
T1: Thread 1; T2: Thread 2; T3: Thread 3
x: Thread is in running state
Figure 3. Ring buffer for default mode
2.2.2 Per-thread mode¶
By specifying option --per-thread in perf command, e.g.
perf record --per-thread test_program
The perf event doesn’t map to any CPUs and is only bound to the profiled process, thus, the perf event’s attributions are:
evsel::cpus::map[0] = { -1 }
evsel::threads::map[] = { pid }
evsel::attr::inherit = 0
In this mode, a single ring buffer is allocated for the profiled thread; if the thread is scheduled on a CPU, the events on that CPU will be enabled; and if the thread is scheduled out from the CPU, the events on the CPU will be disabled. When the thread is migrated from one CPU to another, the events are to be disabled on the previous CPU and enabled on the next CPU correspondingly.
T1 T2 T1
+----+ +-----------+ +----+
CPU0 |xxxx| |xxxxxxxxxxx| |xxxx|
+----+--------------+-----------+----------+----+-------->
| |
| T1 |
| +-----+ |
CPU1 | |xxxxx| |
--|--+-----+----------------------------------|---------->
| | |
| | T1 T3 |
| | +----+ +---+ |
CPU2 | | |xxxx| |xxx| |
--|-----|-----------------+----+--------+---+-|---------->
| | | |
| | T1 | |
| | +--------------+ | |
CPU3 | | |xxxxxxxxxxxxxx| | |
--|-----|--+--------------+-|-----------------|---------->
| | | | |
v v v v v
+-----------------------------------------------------+
| Ring buffer |
+-----------------------------------------------------+
T1: Thread 1
x: Thread is in running state
Figure 4. Ring buffer for per-thread mode
When perf runs in per-thread mode, a ring buffer is allocated for the profiled thread T1. The ring buffer is dedicated for thread T1, if the thread T1 is running, the perf events will be recorded into the ring buffer; when the thread is sleeping, all associated events will be disabled, thus no trace data will be recorded into the ring buffer.
2.2.3 Per-CPU mode¶
The option -C is used to collect samples on the list of CPUs, for
example the below perf command receives option -C 0,2:
perf record -C 0,2 test_program
It maps the perf event to CPUs 0 and 2, and the event is not associated to any PID. Thus the perf event attributions are set as:
evsel::cpus::map[0] = { 0, 2 }
evsel::threads::map[] = { -1 }
evsel::attr::inherit = 0
This results in the session of perf record will sample all threads on CPU0
and CPU2, and be terminated until test_program exits. Even there have tasks
running on CPU1 and CPU3, since the ring buffer is absent for them, any
activities on these two CPUs will be ignored. A usage case is to combine the
options for per-thread mode and per-CPU mode, e.g. the options –C 0,2 and
––per–thread are specified together, the samples are recorded only when
the profiled thread is scheduled on any of the listed CPUs.
T1 T2 T1
+----+ +-----------+ +----+
CPU0 |xxxx| |xxxxxxxxxxx| |xxxx|
+----+--------------+-----------+----------+----+-------->
| | |
v v v
+-----------------------------------------------------+
| Ring buffer 0 |
+-----------------------------------------------------+
T1
+-----+
CPU1 |xxxxx|
-----+-----+--------------------------------------------->
T1 T3
+----+ +-------+
CPU2 |xxxx| |xxxxxxx|
--------------------------+----+--------+-------+-------->
| |
v v
+-----------------------------------------------------+
| Ring buffer 1 |
+-----------------------------------------------------+
T1
+--------------+
CPU3 |xxxxxxxxxxxxxx|
-----------+--------------+------------------------------>
T1: Thread 1; T2: Thread 2; T3: Thread 3
x: Thread is in running state
Figure 5. Ring buffer for per-CPU mode
2.2.4 System wide mode¶
By using option –a or ––all–cpus, perf collects samples on all CPUs
for all tasks, we call it as the system wide mode, the command is:
perf record -a test_program
Similar to the per-CPU mode, the perf event doesn’t bind to any PID, and it maps to all CPUs in the system:
evsel::cpus::map[] = { 0 .. _SC_NPROCESSORS_ONLN-1 }
evsel::threads::map[] = { -1 }
evsel::attr::inherit = 0
In the system wide mode, every CPU has its own ring buffer, all threads are monitored during the running state and the samples are recorded into the ring buffer belonging to the CPU which the events occurred on.
T1 T2 T1
+----+ +-----------+ +----+
CPU0 |xxxx| |xxxxxxxxxxx| |xxxx|
+----+--------------+-----------+----------+----+-------->
| | |
v v v
+-----------------------------------------------------+
| Ring buffer 0 |
+-----------------------------------------------------+
T1
+-----+
CPU1 |xxxxx|
-----+-----+--------------------------------------------->
|
v
+-----------------------------------------------------+
| Ring buffer 1 |
+-----------------------------------------------------+
T1 T3
+----+ +-------+
CPU2 |xxxx| |xxxxxxx|
--------------------------+----+--------+-------+-------->
| |
v v
+-----------------------------------------------------+
| Ring buffer 2 |
+-----------------------------------------------------+
T1
+--------------+
CPU3 |xxxxxxxxxxxxxx|
-----------+--------------+------------------------------>
|
v
+-----------------------------------------------------+
| Ring buffer 3 |
+-----------------------------------------------------+
T1: Thread 1; T2: Thread 2; T3: Thread 3
x: Thread is in running state
Figure 6. Ring buffer for system wide mode
2.3 Accessing buffer¶
Based on the understanding of how the ring buffer is allocated in various modes, this section explains access the ring buffer.
2.3.1 Producer-consumer model¶
In the Linux kernel, the PMU events can produce samples which are stored into the ring buffer; the perf command in user space consumes the samples by reading out data from the ring buffer and finally saves the data into the file for post analysis. It’s a typical producer-consumer model for using the ring buffer.
The perf process polls on the PMU events and sleeps when no events are
incoming. To prevent frequent exchanges between the kernel and user
space, the kernel event core layer introduces a watermark, which is
stored in the perf_buffer::watermark. When a sample is recorded into
the ring buffer, and if the used buffer exceeds the watermark, the
kernel wakes up the perf process to read samples from the ring buffer.
Perf
/ | Read samples
Polling / `--------------| Ring buffer
v v ;---------------------v
+----------------+ +---------+---------+ +-------------------+
|Event wait queue| |data_head|data_tail| |***|***| | |***|
+----------------+ +---------+---------+ +-------------------+
^ ^ `------------------------^
| Wake up tasks | Store samples
+-----------------------------+
| Kernel event core layer |
+-----------------------------+
* : the data is filled by the writer.
Figure 7. Writing and reading the ring buffer
When the kernel event core layer notifies the user space, because
multiple events might share the same ring buffer for recording samples,
the core layer iterates every event associated with the ring buffer and
wakes up tasks waiting on the event. This is fulfilled by the kernel
function ring_buffer_wakeup().
After the perf process is woken up, it starts to check the ring buffers one by one, if it finds any ring buffer containing samples it will read out the samples for statistics or saving into the data file. Given the perf process is able to run on any CPU, this leads to the ring buffer potentially being accessed from multiple CPUs simultaneously, which causes race conditions. The race condition handling is described in the section 2.3.5 Memory synchronization.
2.3.2 Properties of the ring buffers¶
Linux kernel supports two write directions for the ring buffer: forward and backward. The forward writing saves samples from the beginning of the ring buffer, the backward writing stores data from the end of the ring buffer with the reversed direction. The perf tool determines the writing direction.
Additionally, the tool can map buffers in either read-write mode or read-only mode to the user space.
The ring buffer in the read-write mode is mapped with the property
PROT_READ | PROT_WRITE. With the write permission, the perf tool
updates the data_tail to indicate the data start position. Combining
with the head pointer data_head, which works as the end position of
the current data, the perf tool can easily know where read out the data
from.
Alternatively, in the read-only mode, only the kernel keeps to update
the data_head while the user space cannot access the data_tail due
to the mapping property PROT_READ.
As a result, the matrix below illustrates the various combinations of direction and mapping characteristics. The perf tool employs two of these combinations to support buffer types: the non-overwrite buffer and the overwritable buffer.
Mapping mode |
Forward |
Backward |
|---|---|---|
read-write |
Non-overwrite ring buffer |
Not used |
read-only |
Not used |
Overwritable ring buffer |
The non-overwrite ring buffer uses the read-write mapping with forward
writing. It starts to save data from the beginning of the ring buffer
and wrap around when overflow, which is used with the read-write mode in
the normal ring buffer. When the consumer doesn’t keep up with the
producer, it would lose some data, the kernel keeps how many records it
lost and generates the PERF_RECORD_LOST records in the next time
when it finds a space in the ring buffer.
The overwritable ring buffer uses the backward writing with the
read-only mode. It saves the data from the end of the ring buffer and
the data_head keeps the position of current data, the perf always
knows where it starts to read and until the end of the ring buffer, thus
it don’t need the data_tail. In this mode, it will not generate the
PERF_RECORD_LOST records.
2.3.3 Writing samples into buffer¶
When a sample is taken and saved into the ring buffer, the kernel
prepares sample fields based on the sample type; then it prepares the
info for writing ring buffer which is stored in the structure
perf_output_handle. In the end, the kernel outputs the sample into
the ring buffer and updates the head pointer in the user page so the
perf tool can see the latest value.
The structure perf_output_handle serves as a temporary context for
tracking the information related to the buffer. The advantages of it is
that it enables concurrent writing to the buffer by different events.
For example, a software event and a hardware PMU event both are enabled
for profiling, two instances of perf_output_handle serve as separate
contexts for the software event and the hardware event respectively.
This allows each event to reserve its own memory space for populating
the record data.
2.3.4 Reading samples from buffer¶
In the user space, the perf tool utilizes the perf_event_mmap_page
structure to handle the head and tail of the buffer. It also uses
perf_mmap structure to keep track of a context for the ring buffer, this
context includes information about the buffer’s starting and ending
addresses. Additionally, the mask value can be utilized to compute the
circular buffer pointer even for an overflow.
Similar to the kernel, the perf tool in the user space first reads out
the recorded data from the ring buffer, and then updates the buffer’s
tail pointer perf_event_mmap_page::data_tail.
2.3.5 Memory synchronization¶
The modern CPUs with relaxed memory model cannot promise the memory
ordering, this means it’s possible to access the ring buffer and the
perf_event_mmap_page structure out of order. To assure the specific
sequence for memory accessing perf ring buffer, memory barriers are
used to assure the data dependency. The rationale for the memory
synchronization is as below:
Kernel User space
if (LOAD ->data_tail) { LOAD ->data_head
(A) smp_rmb() (C)
STORE $data LOAD $data
smp_wmb() (B) smp_mb() (D)
STORE ->data_head STORE ->data_tail
}
The comments in tools/include/linux/ring_buffer.h gives nice description for why and how to use memory barriers, here we will just provide an alternative explanation:
(A) is a control dependency so that CPU assures order between checking
pointer perf_event_mmap_page::data_tail and filling sample into ring
buffer;
(D) pairs with (A). (D) separates the ring buffer data reading from
writing the pointer data_tail, perf tool first consumes samples and then
tells the kernel that the data chunk has been released. Since a reading
operation is followed by a writing operation, thus (D) is a full memory
barrier.
(B) is a writing barrier in the middle of two writing operations, which makes sure that recording a sample must be prior to updating the head pointer.
(C) pairs with (B). (C) is a read memory barrier to ensure the head pointer is fetched before reading samples.
To implement the above algorithm, the perf_output_put_handle() function
in the kernel and two helpers ring_buffer_read_head() and
ring_buffer_write_tail() in the user space are introduced, they rely
on memory barriers as described above to ensure the data dependency.
Some architectures support one-way permeable barrier with load-acquire
and store-release operations, these barriers are more relaxed with less
performance penalty, so (C) and (D) can be optimized to use barriers
smp_load_acquire() and smp_store_release() respectively.
If an architecture doesn’t support load-acquire and store-release in its
memory model, it will roll back to the old fashion of memory barrier
operations. In this case, smp_load_acquire() encapsulates
READ_ONCE() + smp_mb(), since smp_mb() is costly,
ring_buffer_read_head() doesn’t invoke smp_load_acquire() and it uses
the barriers READ_ONCE() + smp_rmb() instead.
3. The mechanism of AUX ring buffer¶
In this chapter, we will explain the implementation of the AUX ring buffer. In the first part it will discuss the connection between the AUX ring buffer and the regular ring buffer, then the second part will examine how the AUX ring buffer co-works with the regular ring buffer, as well as the additional features introduced by the AUX ring buffer for the sampling mechanism.
3.1 The relationship between AUX and regular ring buffers¶
Generally, the AUX ring buffer is an auxiliary for the regular ring
buffer. The regular ring buffer is primarily used to store the event
samples and every event format complies with the definition in the
union perf_event; the AUX ring buffer is for recording the hardware
trace data and the trace data format is hardware IP dependent.
The general use and advantage of the AUX ring buffer is that it is written directly by hardware rather than by the kernel. For example, regular profile samples that write to the regular ring buffer cause an interrupt. Tracing execution requires a high number of samples and using interrupts would be overwhelming for the regular ring buffer mechanism. Having an AUX buffer allows for a region of memory more decoupled from the kernel and written to directly by hardware tracing.
The AUX ring buffer reuses the same algorithm with the regular ring
buffer for the buffer management. The control structure
perf_event_mmap_page extends the new fields aux_head and aux_tail
for the head and tail pointers of the AUX ring buffer.
During the initialisation phase, besides the mmap()-ed regular ring
buffer, the perf tool invokes a second syscall in the
auxtrace_mmap__mmap() function for the mmap of the AUX buffer with
non-zero file offset; rb_alloc_aux() in the kernel allocates pages
correspondingly, these pages will be deferred to map into VMA when
handling the page fault, which is the same lazy mechanism with the
regular ring buffer.
AUX events and AUX trace data are two different things. Let’s see an example:
perf record -a -e cycles -e cs_etm/@tmc_etr0/ -- sleep 2
The above command enables two events: one is the event cycles from PMU and another is the AUX event cs_etm from Arm CoreSight, both are saved into the regular ring buffer while the CoreSight’s AUX trace data is stored in the AUX ring buffer.
As a result, we can see the regular ring buffer and the AUX ring buffer
are allocated in pairs. The perf in default mode allocates the regular
ring buffer and the AUX ring buffer per CPU-wise, which is the same as
the system wide mode, however, the default mode records samples only for
the profiled program, whereas the latter mode profiles for all programs
in the system. For per-thread mode, the perf tool allocates only one
regular ring buffer and one AUX ring buffer for the whole session. For
the per-CPU mode, the perf allocates two kinds of ring buffers for
selected CPUs specified by the option -C.
The below figure demonstrates the buffers’ layout in the system wide mode; if there are any activities on one CPU, the AUX event samples and the hardware trace data will be recorded into the dedicated buffers for the CPU.
T1 T2 T1
+----+ +-----------+ +----+
CPU0 |xxxx| |xxxxxxxxxxx| |xxxx|
+----+--------------+-----------+----------+----+-------->
| | |
v v v
+-----------------------------------------------------+
| Ring buffer 0 |
+-----------------------------------------------------+
| | |
v v v
+-----------------------------------------------------+
| AUX Ring buffer 0 |
+-----------------------------------------------------+
T1
+-----+
CPU1 |xxxxx|
-----+-----+--------------------------------------------->
|
v
+-----------------------------------------------------+
| Ring buffer 1 |
+-----------------------------------------------------+
|
v
+-----------------------------------------------------+
| AUX Ring buffer 1 |
+-----------------------------------------------------+
T1 T3
+----+ +-------+
CPU2 |xxxx| |xxxxxxx|
--------------------------+----+--------+-------+-------->
| |
v v
+-----------------------------------------------------+
| Ring buffer 2 |
+-----------------------------------------------------+
| |
v v
+-----------------------------------------------------+
| AUX Ring buffer 2 |
+-----------------------------------------------------+
T1
+--------------+
CPU3 |xxxxxxxxxxxxxx|
-----------+--------------+------------------------------>
|
v
+-----------------------------------------------------+
| Ring buffer 3 |
+-----------------------------------------------------+
|
v
+-----------------------------------------------------+
| AUX Ring buffer 3 |
+-----------------------------------------------------+
T1: Thread 1; T2: Thread 2; T3: Thread 3
x: Thread is in running state
Figure 8. AUX ring buffer for system wide mode
3.2 AUX events¶
Similar to perf_output_begin() and perf_output_end()’s working for the
regular ring buffer, perf_aux_output_begin() and perf_aux_output_end()
serve for the AUX ring buffer for processing the hardware trace data.
Once the hardware trace data is stored into the AUX ring buffer, the PMU
driver will stop hardware tracing by calling the pmu::stop() callback.
Similar to the regular ring buffer, the AUX ring buffer needs to apply
the memory synchronization mechanism as discussed in the section
2.3.5 Memory synchronization. Since the AUX ring buffer is managed by the
PMU driver, the barrier (B), which is a writing barrier to ensure the trace
data is externally visible prior to updating the head pointer, is asked
to be implemented in the PMU driver.
Then pmu::stop() can safely call the perf_aux_output_end() function to
finish two things:
It fills an event
PERF_RECORD_AUXinto the regular ring buffer, this event delivers the information of the start address and data size for a chunk of hardware trace data has been stored into the AUX ring buffer;Since the hardware trace driver has stored new trace data into the AUX ring buffer, the argument size indicates how many bytes have been consumed by the hardware tracing, thus
perf_aux_output_end()updates the header pointerperf_buffer::aux_headto reflect the latest buffer usage.
At the end, the PMU driver will restart hardware tracing. During this temporary suspending period, it will lose hardware trace data, which will introduce a discontinuity during decoding phase.
The event PERF_RECORD_AUX presents an AUX event which is handled in the
kernel, but it lacks the information for saving the AUX trace data in
the perf file. When the perf tool copies the trace data from AUX ring
buffer to the perf data file, it synthesizes a PERF_RECORD_AUXTRACE
event which is not a kernel ABI, it’s defined by the perf tool to describe
which portion of data in the AUX ring buffer is saved. Afterwards, the perf
tool reads out the AUX trace data from the perf file based on the
PERF_RECORD_AUXTRACE events, and the PERF_RECORD_AUX event is used to
decode a chunk of data by correlating with time order.
3.3 Snapshot mode¶
Perf supports snapshot mode for AUX ring buffer, in this mode, users only record AUX trace data at a specific time point which users are interested in. E.g. below gives an example of how to take snapshots with 1 second interval with Arm CoreSight:
perf record -e cs_etm/@tmc_etr0/u -S -a program &
PERFPID=$!
while true; do
kill -USR2 $PERFPID
sleep 1
done
The main flow for snapshot mode is:
Before a snapshot is taken, the AUX ring buffer acts in free run mode. During free run mode the perf doesn’t record any of the AUX events and trace data;
Once the perf tool receives the USR2 signal, it triggers the callback function
auxtrace_record::snapshot_start()to deactivate hardware tracing. The kernel driver then populates the AUX ring buffer with the hardware trace data, and the eventPERF_RECORD_AUXis stored in the regular ring buffer;Then perf tool takes a snapshot,
record__read_auxtrace_snapshot()reads out the hardware trace data from the AUX ring buffer and saves it into perf data file;After the snapshot is finished,
auxtrace_record::snapshot_finish()restarts the PMU event for AUX tracing.
The perf only accesses the head pointer perf_event_mmap_page::aux_head
in snapshot mode and doesn’t touch tail pointer aux_tail, this is
because the AUX ring buffer can overflow in free run mode, the tail
pointer is useless in this case. Alternatively, the callback
auxtrace_record::find_snapshot() is introduced for making the decision
of whether the AUX ring buffer has been wrapped around or not, at the
end it fixes up the AUX buffer’s head which are used to calculate the
trace data size.
As we know, the buffers’ deployment can be per-thread mode, per-CPU mode, or system wide mode, and the snapshot can be applied to any of these modes. Below is an example of taking snapshot with system wide mode.
Snapshot is taken
|
v
+------------------------+
| AUX Ring buffer 0 | <- aux_head
+------------------------+
v
+--------------------------------+
| AUX Ring buffer 1 | <- aux_head
+--------------------------------+
v
+--------------------------------------------+
| AUX Ring buffer 2 | <- aux_head
+--------------------------------------------+
v
+---------------------------------------+
| AUX Ring buffer 3 | <- aux_head
+---------------------------------------+
Figure 9. Snapshot with system wide mode