Difference between revisions of "Choosing a processor for a build farm"

Revision as of 02:10, 7 February 2016

Overview

Build farms are network clusters of nodes with high CPU performance which are dedicated to build software. The general approach consists in running tools like "distcc" on the developer's workstation, which will delegate the job of compiling to all available nodes. The CPU architecture is irrelevant here since cross-compilation for any platform is involved anyway.

Build farms are only interesting if they can build faster than any commonly available, cheaper solution, starting with the developer's workstation. Note that developers workstations are commonly very powerful, so in order to provide any benefit, a cluster aiming at being faster than this workstation still needs to be affordable.

In terms of compilation performance, the metrics are lines-of-code per second per dollar (performance) and lines of code per joule (efficiency). A number of measurements were run on various hardware, and this research is still going. To sum up observations, most interesting solutions are in the middle range. Too cheap devices have too small CPUs or RAM bandwidth, and too expensive devices optimise for areas irrelevant to build speed, or have a pricing model that exponentially follows performance.

Hardware considerations

Nowadays, most processors are optimised for higher graphics performance. Unfortunately, it's still not possible to run GCC on the GPU. And we're wasting transistors, space, power and thermal budget in a part that is totally unused in a build farm. Similarly, we don't need a floating point unit in a build farm. In fact, if some code to be built uses a lot of floating point operations, the compiler will have to use floating point as well to deal with constants and some optimisations, but such code is often marginal in a whole program, let alone distribution (except maybe in HPC environments).

Thus, any CPU with many cores at a high frequency and high memory bandwidth may be eligible for testing, even if there's neither FPU nor GPU.

Methodology

First, software changes a lot. This implies that comparing numbers between machines is not always easy. It could be possible to insist on building an outdated piece of code with an outdated compiler, but that would be pointless. Better build modern code that the developer needs to build right now, with the compiler he wants to use. As a consequence, a single benchmark is useless, it always needs to be compared to one run on another system with the same compiler and code. After all, the purpose of building a build farm is to offload the developer's system so it makes sense to use this system as a reference and compare the same version of toolchain on the device being evaluated.

Porting a compiler to another machine

The most common operation here is what is called a Canadian build. It consists in building on machine A a compiler aimed at running on machine B to produce code for machine C. For example a developer using an x86-64 system could build an ARMv7 compiler producing code for a MIPS platform. Canadian builds sometimes fail because of bugs in the compiler's build system which sometimes mixes variables between build, host or target. For its defense, the principle is complex and detecting unwanted sharing there is even more difficult than detecting similar issues in more common cross-compilation operations.

In case of failure, it can be easier to proceed in two steps :

• canadian build from developer's system to tested device for tested device. This results in a compiler that runs natively on the test device.
• native build on the test device of a cross-compiler for the target system using the previously built compiler.

Since that's a somewhat painful (or at least annoying) task, it makes sense to back up resulting compilers and to simply recopy it to future devices to be tested if they use the same architecture.

Tests

Conditions

This test consisted in building haproxy-git-6bcb0a84 using gcc-4.7.4, producing code for i386. In all tests, no I/O operations were made because the compiler, the sources and the resulting binaries were all placed in a RAM disk. The APU and the ARMs were running from a Formilux RAM disk.

Test method

HAProxy's sources matching Git commit ID 6dcb0a84 are extracted into /dev/shm. A purposely built toolchain based on gcc-4.7.4 and glibc-2.18 is extracted in /dev/shm as well. The "make" utility is installed on the system if not present. HAProxy is always built in the same conditions, TARGET is set to "linux2628", EXTRA is empty, CC points to the cross-compiler, and LD is set to "#" to disable the last linking phase which cannot be parallelized. Then the build is run at least 3 times with a parallel setting set sweeping 1 to the number of CPU cores, in powers of two, and the shortest build time is noted. Example :

root@t034:~# cd /dev/shm
root@t034:shm# tar xf haproxy-6dcb0a84.tar
root@t034:shm# tar xf i586-gcc47l_glibc218-linux-gnu-full-arm.tgz
root@t034:shm# cd haproxy
root@t034:haproxy# make clean
root@t034:haproxy# time make -j 4 TARGET=linux2628 EXTRA= CC=../i586-*-gnu/bin/i586-*-gcc LD='#'
...
real  25.032s
user  1m33.770s
sys   0m1.690s

Metrics

There are multiple possible metrics to compare one platform to another. The first one obviously is the build time. Another one is the inverse of the build time, it's the number of lines of code ("loc") compiled per second ("loc/s"). Then this one can be declined to loc/s/core, loc/s/GHz, loc/s/watt, loc/s/euro, etc. So the first thing to do is to count the number of lines of code that are compiled. By simply replacing "-c" with "-S" in haproxy's Makefile, the resulting ".o" files only contain pre-processed C code with lots of debugging information beginning with "#", and many empty lines. We'll simply get rid of all of this and count the number of real lines the compiler has to run through :

$ make clean
$ find . -name '*.o' | xargs cat | grep -v '^#' | grep -v '^$' | wc -l
0
$ make -j 4 TARGET=linux2628 EXTRA= CC=/w/dev/i586-gcc47l_glibc218-linux-gnu/bin/i586-gcc47l_glibc218-linux-gnu-gcc LD='#'
...
$ find . -name '*.o' | xargs cat | grep -v '^#' | grep -v '^$' | wc -l
458870

OK then we have an easily reproducible reference value : the test set consists of 458870 lines of code.

Devices

Machines involved in this test were 32 & 64 bit x86 as well as 32-bit ARMv7 platforms :

Results

And the results are presented below in build time for various levels of parallel build.

Analysis on 2014/08/05

The Core2quad is outdated. It's exactly half as powerful as the new core i5 despite running at sensibly the same frequency. ARMs do not perform that well here. The XP-GP achieves the performance of one core of the C2Q using all of its 4 cores. Since it's running at half the frequency, we can consider that each core of this Armada-XP chip delivers approximately half of the performance of a C2Q at the same frequency in this workload. The Atom in the EEE-PC, despite a slightly higher frequency than the Armada-XP, is not even able to catch up with it. The APU platform is significantly more efficient at similar frequency than the Atom, given that it delivers per core at 1.0 GHz the same performance as the Atom at 1.86 GHz. However the atom can use its HyperThreading to save 25 extra percent of build time and reach a build time that the APU cannot achieve.

The conclusion here is that low-end x86 CPUs such as the Core i3 3217U at 1.8 GHz should still be able to achieve half of the Core i5's performance, or be on par with the C2Q, despite consuming only 17W instead of the C2Q's 77W. All x86 machines are still expensive because you need to add memory and sometimes a small SSD if you cannot boot them over the network. Given the arrival of new Cortex A17 at 2+ GHz supposed to be 60% faster than A9s clock-for-clock (Armada XP's PJ4B core is very similar to A9), there could be some hope to see interesting improvements there. If an A17 could perform as half of the i5 for quarter of its price (or half the price of a fully-equiped low-end i3), it would mean a build farm based on these devices would not be stupid.

Analysis on 2015/01/18

As expected, the Cortex A17 running at the heart of RK3288 shows a very good performance, and a single core performs about 60% faster than Armada XP's clock for clock, resulting in each core being 1.92 times faster thanks to the higher frequency. This is visible in the single core test which shows exactly the same speed as two cores on XP-GP, and the two-core test which is almost twice as fast on the RK3288. However, this 4-core CPU doesn't scale well to 3 nor 4 processes. The very likely reason is that not only the RAM is limited to a 32-bit bus, but it runs at 1066 MHz only. In comparison, the Armada XP is powered by 1600 MHz in 64-bit, resulting in exactly 3 times the bandwidth. The 4-core run on the RK3288 was only 67% faster than the 2-core one. Linear scaling should have shown around 21 seconds for 4 processes instead of 25. It is possible that other devices running faster DDR3/DDR3L and more channels would not experience this performance loss. That said, this device is by far the fastest of all non-x86 devices here and is even much faster than all low-end CPUs tested so far. 4 cores of RK3288 give approximately the same build power as one core of an intel core i5 at 3 GHz. The device is cheap (less than 75 EUR shipping included) and can really compete with lwo-end PCs which still require addition of RAM and storage. For less than 300 EUR, you get the equivalent of four 3GHz intel cores with 8 GB of RAM, and it is completely fanless.

Comparing clock speed and core count, the Cortex A7 in the cubietruck is the slower, but it's still more or less on par with the Sheeva core in the Armada XP which is normally comparable to a Cortex A9. Here it suffers from a very low frequency (1.008 GHz) but possibly the big.Little designs offering it as a high-frequency companion to an even faster Cortex A15/A17 can bring some benefits. The Cortex A17 cores in the T034 are 26% faster than the Atom 2600 cores in the EEEPC. However, the Atom manages to catch up when it uses Hyperthreading. It's said that newer Atoms are about 20% faster so they could catch up with it without Hyperthreading, and manage to be twice as fast in their quad-core version.

The intel Core as found in the Core i5 is performing very well, but hyperthreading brings little benefit here (13%). Thus a CPU should be chosen with real cores (eg: newer code-core i5 vs dual-core quad-thread i3). The Phenom is performing quite well also, especially if we consider it's an old platform of the same generation as the Core2. Also, newer CPUs are proposed with many cores (4-8) and a high frequency (4+ GHz) leaving hopes for a very fast build platform.

Concerning the costs, An AMD diskless platform can be built for about 400 EUR with eight 4 GHz cores, 4 GB RAM, a motherboard and a power supply. Such a machine could theorically deliver around 160 kloc/s. The ARM-based T034 would require 9 machines to match that result, and would cost about 675 EUR + the switch ports. However it would take 10 times less space and would be fanless, silent and eat a third of the power. It seems reasonable to expect that a big.Little machine running 4 cortex 15 cores at 2 GHz and 4 cortex A7 cores at 1.3 GHz could reach 25 kloc/s. Such a machine could be made for around 150 EUR using a Cubietruck 4 or an Odroid XU3 board. But compared to the other solutions, that's even more expensive (around 900 EUR for 160 kloc/s). Maybe entry-level Atom-based motherboard with an external power supply can compete with the AMD if one manages to find them at an affordable price.

In this test, the AMD was the fastest of all systems, and the ARM was the fastest fanless system and also the one delivering the highest throughput per cubic centimeter.

Analysis on 2015/02/07

The PCduino8-uno is quite an interesting device. For $49, you get 8 cores running at 1.8 GHz (it's advertised as 2.0 GHz but that's wrong, cpu-freq refuses any frequency above 1.8 GHz). The processor is very similar to the A83T. It supports DVFS (dynamic voltage and frequency scaling) and automatically adjusts its frequency based on the temperature. This is a benefit for hardware vendors who can easily overclock CPUs and advertise the highest possible frequency, at the expense of the consumers who find them slow because these devices throttle themselves. The PCduino8-uno is no exception, after 2 or 3 seconds, it slows down to only 4 cores running at 480 MHz, and completely stops the 4 other ones. The temperature thresholds are extremely low (it starts to throttle above 50 degrees). This thermal throttling can be disabled, but at exactly 100 degrees, the device will shutdown, making it really useless for anything. By writing to the proper CPU register, it's possible to also diable the thermal shutdown. The CPU then runs find at up to 121 degrees, and froze at 128. Installing a 4cmx4.5cm heatsink on this device was not easy, but adhesive heatsinks of 3.5x3.5cm can easily be found on the net. With this heatsink, the temperature rarely exceeds 80 degrees. This combined with a disabled thermal throttling is enough to let the device *really* work with all 8 cores at 1.8 GHz.

The test with 4 processes shows that the memory bandwidth starts to limit the build performance. According to the datasheets, the DRAM bus is only 32 bits, which starts to be a bit short for 8 build processes in parallel. No information was found on the DRAM frequency though it's expected to be between 800 and 1066 MT/s.

The test with 8 processes exhibited an issue that never appeared in previous tests and which is worth considering for future hardware choices. It happens that haproxy contains 4 very large C files, each consuming between 8 and 20 seconds to build. So when these files are picked in the middle of the build process, they can be left alone on one or two cores with the other ones idle waiting for them to finish. The only way to improve the situation is to force these files to be built first so that the other smaller ones can be built in parallel. Regardless on the number of cores or available machines, it indicates that the build time using such hardware will never go below 20 seconds.

This shows the importance of raw performance. It's also interesting to note that at similar frequency, the RK3288 in the T034 is exactly as fast as the H8 in the PCduino8-uno but with half of the cores. Thus it will deliver the same build performance with half of the latency (ie with enough nodes it will be possible to go down to 10 seconds instead of 20). But for workloads involving many small files, the PCDuino8-uno is 30% cheaper, though it requires some hardware adjustments.

Other very cheap CPUs exist, such as the H3 (quad core 1.2 GHz). Some are sold overclocked at 1.6 but cannot sustain this frequency when used. Users report that 1.2 is the maximum stable frequency with a heatsink. When the price of a power supply, a microSD card, and a switch port are added, they're probably not interesting anymore, being 3 times slower than the H8 at 1.8 GHz for around $10-$15.

New tests with distcc on the Linux kernel on 2015/02/07

Since we don't have enough small files in haproxy to maintain all cores busy during most of the time, a new test was run with distcc running on the t430, involving various combinations of itself, the pcduino8-uno and the t034.

A first test showed that the t034 would periodically segfault on the Linux kernel and refuse to run distccd on the Android kernel, apparently due to selinux preventing a non-root process from creating a socket. The crashes were caused by the Linux kernel running the memory at 456 MHz while the Android kernel was running it at 396 MHz. A new device tree was made to fix this and to also push the CPU to 1.992 GHz.

As can be seen, the cortex A17 in t034 delivers exactly the same build time with half of the core as the cortex A7 in the pcduino8-uno, so for this workload both machines are interchangeable, thus the pcduino8 is cheaper for the same level of performance, and that these two machines combined achieve almost the same level of performance as the t430s alone. When the t430s is used only by distcc, it's pretty clear that it can easily handle about 6 such machines before being saturated. Thus it should be possible to make the build time go as low as about 45 seconds with 6 machines (it's 2mn09 natively). 6 pcduino8 will cost about $300 plus the microSD cards and the heatsinks and power supplies. This can be cheaper than an x86 equivalent, consume less power and remain fanless. It is expected that the Odroid XU4 board would deliver approximately 1.5 times the performance of these machines for 1.5 times the cost of the pcduino8. It would also have a much lower latency than the pcduino8, making it suitable to build projects using large files. Such an option might be really worth considering in the future. New cheap boards involving cortex A53 (64-bit) still run at too low a frequency to be of any use here.

By making use of the "pump" feature of distcc, it might be possible to further increase the number of nodes, at the expense of a bit more constraints.