* Performance Measurement
{anchor: Perf_Measurement}

** VirtualGL's Built-In Profiling System

The easiest way to uncover bottlenecks in VirtualGL's image pipeline is to
set the ''VGL_PROFILE'' environment variable to ''1'' on both server and
client.  (Passing an argument of ''+pr'' to ''vglrun'' on the server has the same
effect.)  This will cause VirtualGL to measure and report the throughput of
various stages in the pipeline.  For example, here are some measurements from a
dual Pentium 4 server communicating with a Pentium III client on a 100-megabit
LAN:

	Server :: {:}
	#Verb: <<---
	Readback   - 43.27 Mpixels/sec - 34.60 fps
	Compress 0 - 33.56 Mpixels/sec - 26.84 fps
	Total      -  8.02 Mpixels/sec -  6.41 fps - 10.19 Mbits/sec (18.9:1)
	---

	Client :: {:}
	#Verb: <<---
	Decompress - 10.35 Mpixels/sec -  8.28 fps
	Blit       - 35.75 Mpixels/sec - 28.59 fps
	Total      -  8.00 Mpixels/sec -  6.40 fps - 10.18 Mbits/sec (18.9:1)
	---

The total throughput of the pipeline is 8.0 Megapixels/sec, or 6.4 frames/sec,
indicating that our frame is 8.0 / 6.4 = 1.25 Megapixels in size (a little less
than 1280 x 1024 pixels.)  The readback and compress stages, which occur in
parallel on the server, are obviously not slowing things down, and we're only
using 1/10 of our available network bandwidth.  Looking at the client, however,
we discover that its slow decompression speed (10.35 Megapixels/second) is the
primary bottleneck.  Decompression and blitting on the client cannot be done in
parallel, so the aggregate performance is the harmonic mean of the
decompression and blitting rates:
__[1/ (1/10.35 + 1/35.75)] = 8.0 Mpixels/sec__.  In this case, we could improve
the performance of the whole system by simply using a client with a faster CPU.

	!!! This example is meant to demonstrate how the client can sometimes be
	the primary impediment to VirtualGL's end-to-end performance.  Using "modern"
	hardware in both the server and client, VirtualGL can easily stream 50+
	Megapixels/sec across a LAN, as of this writing.

** Frame Spoiling
{anchor: Frame_Spoiling}

By default, VirtualGL will only transport a frame if the image transport is
ready to receive it.  If VirtualGL detects that the 3D application has finished
rendering a new frame but there are already frames waiting in the queue to be
transported, then those untransported frames are dropped ("spoiled"), and the
new frame is promoted to the head of the queue.  This prevents a backlog of
frames on the server, which would cause a perceptible delay in the
responsiveness of interactive 3D applications.  However, when running
non-interactive 3D applications (particularly benchmarks), frame spoiling
should always be disabled.  With frame spoiling disabled, the server will
render frames only as quickly as VirtualGL can transport those frames, which
will conserve server resources as well as allow OpenGL benchmarks to accurately
measure the end-to-end performance of VirtualGL.  With frame spoiling enabled,
OpenGL benchmarks will report meaningless data, since the rate at which the
server can render frames is decoupled from the rate at which VirtualGL can
transport those frames.

In most X proxies (including VNC), there is effectively another layer of frame
spoiling, since the rate at which the X proxy can send frames to the client is
decoupled from the rate at which VirtualGL can draw rendered frames into the X
proxy.  Thus, even if frame spoiling is disabled in VirtualGL, OpenGL
benchmarks will still report inaccurate data if they are run in such X proxies.
TCBench, described below, provides a limited solution to this problem.

To disable frame spoiling, set the ''VGL_SPOIL'' environment variable to ''0''
on the VirtualGL server or pass an argument of ''-sp'' to ''vglrun''.  See
{ref prefix="Section ": VGL_SPOIL} for further information.

** VirtualGL Diagnostic Tools

VirtualGL includes several tools that can be useful for diagnosing performance
problems with the system.

*** NetTest
#OPT: noList! plain!

NetTest is a network benchmark that uses the same network I/O classes as
VirtualGL.  It can be used to test the latency and throughput of any TCP/IP
connection.  ''nettest'' is installed in {file: /opt/VirtualGL/bin} by default.
For Windows users, a native Windows version of NetTest is included in the
{file: VirtualGL-Utils} package, which is distributed alongside VirtualGL.

To use NetTest, first start up the NetTest server on one end of the
connection:

#Verb: <<---
nettest -server
---

Next, start the NetTest client on the other end of the connection:

#Pverb: <<---
nettest -client __server__
---

Replace __''server''__ with the hostname or IP address of the machine on which
the NetTest server is running.

The NetTest client will produce output similar to the following:

#Pverb: <<---
TCP transfer performance between localhost and __server__:

Transfer size  1/2 Round-Trip      Throughput      Throughput
(bytes)                (msec)    (MBytes/sec)     (Mbits/sec)
1                    0.093402        0.010210        0.085651
2                    0.087308        0.021846        0.183259
4                    0.087504        0.043594        0.365697
8                    0.088105        0.086595        0.726409
16                   0.090090        0.169373        1.420804
32                   0.093893        0.325026        2.726514
64                   0.102289        0.596693        5.005424
128                  0.118493        1.030190        8.641863
256                  0.146603        1.665318       13.969704
512                  0.205092        2.380790       19.971514
1024                 0.325896        2.996542       25.136815
2048                 0.476611        4.097946       34.376065
4096                 0.639502        6.108265       51.239840
8192                 1.033596        7.558565       63.405839
16384                1.706110        9.158259       76.825049
32768                3.089896       10.113608       84.839091
65536                5.909509       10.576174       88.719379
131072              11.453894       10.913319       91.547558
262144              22.616389       11.053931       92.727094
524288              44.882406       11.140223       93.450962
1048576             89.440702       11.180592       93.789603
2097152            178.536997       11.202160       93.970529
4194304            356.754396       11.212195       94.054712
---

We can see that the throughput peaks at about 94 megabits/sec, which is pretty
good for a 100-megabit connection.  We can also see that, for small transfer
sizes, the round-trip time is dominated by latency.  The "latency" is
the same thing as the one-way (1/2 round-trip) transit time for a zero-byte
packet, which is about 93 microseconds in this case.

*** CPUstat
#OPT: noList! plain!

CPUstat is available only for Linux and is installed in the same place as
NetTest ({file: /opt/VirtualGL/bin} by default.)  It measures the average,
minimum, and peak usage for all CPU cores combined and for each CPU core
individually.  On Windows, this same functionality is provided in the Windows
Performance Monitor, which is part of the operating system.  On Solaris, the
same data can be obtained using the ''vmstat'' program.

CPUstat measures the CPU usage over a given sample period (a few seconds) and
continuously reports how much each CPU core was utilized since the last sample
period.  Output for a particular sample looks something like this:

#Verb: <<---
ALL :  51.0 (Usr= 47.5 Nice=  0.0 Sys=  3.5) / Min= 47.4 Max= 52.8 Avg= 50.8
cpu0:  20.5 (Usr= 19.5 Nice=  0.0 Sys=  1.0) / Min= 19.4 Max= 88.6 Avg= 45.7
cpu1:  81.5 (Usr= 75.5 Nice=  0.0 Sys=  6.0) / Min= 16.6 Max= 83.5 Avg= 56.3
---

The first column indicates what percentage of time the CPU core was active
since the last sample period.  This is then broken down into what percentage of
time the CPU core spent running user, nice, and system/kernel code.  ''ALL''
indicates the average utilization across all CPU cores since the last sample
period.  ''Min'', ''Max'', and ''Avg'' indicate a running minimum, maximum, and
average of all samples since CPUstat was started.

Generally, if a 3D application's CPU usage is fairly steady, then you can run
CPUstat for a bit and wait for the ''Max'' and ''Avg'' values in the ''ALL''
category to stabilize, and that will tell you the application's peak and
average CPU utilization.

*** TCBench
#OPT: noList! plain!

TCBench was born out of the need to compare VirtualGL's performance to that of
other thin client software, some of which had frame spoiling features that
could not be disabled.  TCBench measures the frame rate of a thin client system
as seen from the client's point of view.  It does this by attaching to one of
the windows on the client and continuously reading back a small area at the
center of the window.  While this may seem to be a somewhat non-rigorous test,
experiments have shown that, if care is taken to ensure that the 3D application
is updating the center of the window with every frame (such as in a spin
animation), TCBench can produce quite accurate results.  It has been sanity
checked with VirtualGL's internal profiling mechanism and with a variety of
system-specific techniques, such as monitoring redraw events on the client's
windowing system.

TCBench is installed in {file: /opt/VirtualGL/bin} by default.  For Windows
users, a native Windows version of TCBench is included in the
{file: VirtualGL-Utils} package, which is distributed alongside VirtualGL.  Run
''tcbench'' from the command line, and it will prompt you to click in the
window you want to benchmark.   That window should already have an automated
animation of some sort running before you launch TCBench.  Note that GLXSpheres
(see below) is an ideal benchmark to use with TCBench, since GLXSpheres draws a
new sphere to the center of its window every time it renders a frame.

#Verb: <<---
tcbench -?
---

lists the relevant command-line arguments, which can be used to adjust the
benchmark time, the sampling rate, and the x and y offset of the sampling area
within the window.

*** GLXSpheres
#OPT: noList! plain!

GLXSpheres is a benchmark that produces very similar images to nVidia's
(long-discontinued) SphereMark benchmark.  In the early days of VirtualGL's
existence, it was discovered (quite by accident) that SphereMark was a pretty
good test of VirtualGL's end-to-end performance, because that benchmark
generated images with about the same proportion of solid color, and similar
frequency components, to the images generated by volume visualization
applications.

Thus, the goal of GLXSpheres was to create an open source Un*x version of
SphereMark (SphereMark was for Windows only) completely from scratch.
GLXSpheres does not use any code from the original benchmark, but it does
attempt to mimic the visual output of the original as closely as possible.
GLXSpheres lacks some of the advanced rendering features of the original, such
as the ability to use vertex arrays, but since GLXSpheres was primarily
designed as a benchmark for VirtualGL, display lists are more than fast enough
for that purpose.

GLXSpheres has some additional modes that its predecessor lacked, modes that
are designed specifically to test the performance of various VirtualGL
features:

	Stereographic rendering (''glxspheres -s'') :: {:}

	Immediate mode rendering (''glxspheres -m'') :: Want to really see the
	benefit of VirtualGL?  Run ''glxspheres -m'' over a remote X connection, then
	run ''vglrun -sp glxspheres -m'' over the same connection and compare.
	Immediate mode does not use display lists, so when immediate-mode OpenGL is
	rendered indirectly (over a remote X connection), this causes every OpenGL
	command to be sent as a separate network request to the X server ... with
	every frame.  Many 3D applications do not use display lists-- because the
	geometry they are rendering is dynamic, or for other reasons-- so this test
	models how such applications might perform when displayed remotely without
	VirtualGL.

	Interactive mode (''glxspheres -i'') :: In interactive mode, GLXSpheres will
	wait to render a frame until it receives a mouse event.  Continuously
	dragging the mouse in the window should produce a steady frame rate, and this
	frame rate is a reasonable model of the frame rate that you can achieve when
	running interactive 3D applications with VirtualGL.  Comparing this
	interactive frame rate (''vglrun glxspheres -i'') with the non-interactive
	frame rate (''vglrun -sp glxspheres'') allows you to quantify the effect of
	network latency on the performance of interactive applications in a VirtualGL
	environment.

GLXSpheres is installed in {file: /opt/VirtualGL/bin} by default.  64-bit
VirtualGL builds name this program ''glxspheres64'' so as to allow both a
64-bit and a 32-bit version of GLXSpheres to be installed on the same system.

	!!! This version of VirtualGL also provides an EGL/X11 equivalent of
	GLXSpheres (EGLXSpheres), which works identically to GLXSpheres except for
	the absense of modes (including stereographic rendering) that the EGL API
	does not support.
