When doing web development I will occasionally connect my local code base to a remote SQL server via SSH.
This adds enough latency to be noticeable and I’ve found pages that were “OK” in prod that were unbearable in my local environment. Most of the time it was N+1 queries. Sometimes it was a cache that wasn’t working as intended. Sometimes it simply was a feature that “looked cool” but offered no value.
I’m not sure if there is a proxy that would do this locally but I’ve found it invaluable.
I do networked game development on Windows and I've found the clumsy program to be very valuable to simulate adverse network conditions. You can set it up to simulate arbitrary network latency, packet loss and so forth.
I’m not sure if you’re saying the latency was introduced in client <-> server hops or server <-> db hops, but chrome dev tools (I’m sure all browsers too) can simulate different network conditions with a few clicks! Useful for something similar to what you’ve said, but in the end I think you meant server <-> db latency is what you want to inject
In my opinion, NOP and MOV, which are recommended in TFA for slowing down, are the worst possible choices.
The authors have tested a rather obsolete CPU, with a 10-year-old Skylake microarchitecture, but more recent Intel/AMD CPUs have special optimizations for both NOP and MOV, executing them at the renaming stage, well before the normal execution units, so they may appear to have been executed in zero time.
For slowing down, one could use something really slow, like integer division. If that would interfere with the desired register usage, other reliable choices would be add with carry or perhaps complement carry flag. If it is not desired to modify the flags, one can use a RORX instruction for multiple bit rotation (available since Haswell, but not in older Atom CPUs), or one could execute BSWAP (available since 1989, therefore it exists in all 64-bit CPUs, including any Atom).
They care about exact slowdown factors, and they don’t like the instructions that use any of the CPU’s execution resources because the program’s real work interferes with the slowdown factor making it harder to control.
The NOPs in effect use up a small fraction of the instruction decode bandwidth, and if they insert enough NOPs they can reduce the number of real instructions that are issued per cycle and slow down the program with a fine degree of precision.
Many recent CPUs eliminate NOPs without executing them, so using NOP may work on a CPU but be ineffective on another.
A double BSWAP is equivalent with a NOP, but no existing CPU does any special optimization for it and it is very unlikely that any future CPU would do something special with it, as it is mostly obsolete (nowadays MOVBE is preferable instead of it).
NOP cannot ensure a certain slow-down factor, except on exactly the same model of CPU.
> Many recent CPUs eliminate NOPs without executing them
The elimination will still take some amount of time, and the smaller this amount is, the better, because it allows dialing in the slowdown more precisely. Of course how many NOPs you need for a particular basic block needs to be measured on the CPU you're profiling on, but that's also the case if you use a more consistently slow instruction, since the other instructions won't necessarily maintain a consistent speed relative to it.
That's not necessarily true. Many modern CPUs have uop caches, and it's reasonable to assume implementations will eliminate NOPs before placing lines in the uop cache. A hit on the uop cache loses any slowdown you hoped to achieve since now the NOPs are neither fetched nor decoded.
That's an interesting question, because the uop cache is filled early in the pipeline, but zero-idioms are only detected around the rename stage.
They might be able to skip a plain 0x90, but something like mov rax, rax would still emit a uop for the cache, before being eliminated later in rename. So at best it would be a fairly limited optimization.
It's also nice because rename is a very predictable choke point, no matter what the rest of the frontend and the backend are busy doing.
From what I understand if you really want to mess with some performance stuff just throw a random AVX or even better AVX512 workload in the critical path of all cores. Could be something really simple, like expanding out a normal cmov, jmp statement, or just regular integer addition to be needlessly AVX involved. Depending on the CPU this can apparently downclock the core, change the power delivery, and potentially result in pipeline stalls on other cores on the same die.
There's a whole niche in cryptography called verifiable delay functions (VDFs) if you want a big rabbit hole to go down.
The idea is that these are unavoidably slow and preferably non-parallelizable to compute one way, but fast or near instantaneous to verify the result. Examples include the Weslowski VDFs based on similar math to RSA, MIMC, and the use of zero knowledge proofs to provide proof of slow computations.
For this purpose though, computation should be minimized to avoid spilling registers and having more complicated performance side effects than just the intended direct delay.
RDTSC or RDTSCP, like also CPUID, work certainly very well for slowing down a program.
However, like integer division, they may clobber registers that the program wants to use for other purposes.
For great slow-downs when the clobbered registers do not matter, I think that CPUID is the best, as it serializes the execution and it has a long execution time on all CPUs.
For small slow-downs I think that BSWAP is a good choice, as it modifies only 1 arbitrary register without affecting the flags, and it also is a less usual instruction so it is unlikely that it will ever receive special optimizations, like NOP and MOV.
However, multiple BSWAPs must be used, to occupy all available execution ports, otherwise if there is any execution port not occupied by the rest of the program the BSWAP may be executed concurrently, not requiring any extra time.
The purpose of the Turbo button on PCs was indeed to slow it down, originally to the 4.77 MHz of the original 8088, for compatibility. For marketing reasons it was designated “turbo” instead of something indicating a slow-down. Turbo mode was always the default, since that was the standard speed of the respective CPU.
> we can estimate whether an optimization is beneficial before implementing it. Coz simulates it by slowing down all other program parts. The part we think might be optimizable stays at the same speed it was before, but is now virtually sped up, which allows us to see whether it gives enough of a benefit to justify a perhaps lengthy optimization project.
Really interesting idea. I suppose the underlying assumption is that speeding up a function might reveal a new performance floor that is much higher than the speedup would suggest. Spending time to halve a function's time only to discover that overall processing time only decreased slightly because another function is now blocking would indeed be quite bad.
Not sure how this handles I/O, or if it is enough to simply delay the results of I/O functions to simulate decreased bandwidth or increased latency.
After a recent upgrade, I've been pondering how to spot test optimizations on my workstation, which is now much faster than the average target system. (Keeping a slow computer around and continually moving my code to it would be effective, but inconvenient.)
My first idea is to taskset the code to a particular CPU core, and see if linux will let me put that core in a low frequency mode. Has anyone here done this on AMD hardware? If so, were the available frequencies slow enough to be useful for this purpose?
Kind of weird that NOP actually slows down the pipeline, as I'd think that would be the easiest thing to optimize out of the pipeline, unless instruction fetch is one of the main limiting factors. Is it architecturally defined that NOP will slow down execution?
I think it would be easy, but still not worth the transistors. Think of it: what programs contain lots of NOPs? Who, desiring to write a fast program, sprinkles their code with NOPs?
It’s not worth optimizing for situations that do not occur in practice.
The transistors used to detect register clearing using XOR foo,foo, on the other hand, are worth it, as lots of code has that instruction, and removing the data dependency (the instruction technically uses the contents of the foo register, but its result is independent of its value) can speed up code a lot.
On CPUs with variable instruction length, like the Intel/AMD CPUs, many programs have a lot of NOPs, which are inserted by the compiler for instruction alignment.
However those NOPs are seldom executed frequently, because most are outside of loop bodies. Nevertheless, there are cases when NOPs may be located inside big loops, in order to align some branch targets to cache line boundaries.
That is why many recent Intel/AMD CPUs have special hardware for accelerating NOP execution, which may eliminate the NOPs before reaching the execution units.
It depends on the CPU. On some CPUs a NOP might take the same time as an ADD and it might have the same throughput per clock cycle as ADD.
However, there are CPUs among the Intel/AMD CPUs that can execute up to a certain number of consecutive NOPs in zero time, i.e. they are removed from the instruction stream before reaching the execution units.
In general, no instruction set architecture specifies the time needed to execute an instruction. For every specific CPU model you must search its manual to find the latency and throughput for the instruction of interest, including for NOPs.
Some CPUs, like the Intel/AMD CPUs, have multiple encodings for NOP, with different lengths in order to facilitate instruction alignment. In that case the execution time may be not the same for all kinds of NOPs.
I think so, as in "make sure all other stuff has run before calling the NOP finished". Otherwise, it would just skip past it and it would have no effect if placed in a loop, so it would be eating memory for no use at all.
Taking this approach seems to be effective at surfacing issues that otherwise wouldn't show up in regular testing. I could see this being useful if it was standardized to help identify issues before they hit production.
I fondly remember using a program called MoSlow to slow down older DOS games that were used to using every cycle they could get, and so were unplayable when I got a Pentium computer. I joked about how I wish I could find the corresponding "MoFast" program.
Causal profiling is something I infrequently use but I have been able to apply it outside of micro benchmarks in an orchestration system.
If you have a business process that is complex and owned by many different groups then causal profiling can be a good tool for dealing with the complexity. For large workflows in particular this can be powerful as the orchestration owner can experiment without much coordination (other than making sure the groups know/agree that some processing might be delayed).
This adds enough latency to be noticeable and I’ve found pages that were “OK” in prod that were unbearable in my local environment. Most of the time it was N+1 queries. Sometimes it was a cache that wasn’t working as intended. Sometimes it simply was a feature that “looked cool” but offered no value.
I’m not sure if there is a proxy that would do this locally but I’ve found it invaluable.
https://github.com/Shopify/toxiproxy
https://jagt.github.io/clumsy/
The authors have tested a rather obsolete CPU, with a 10-year-old Skylake microarchitecture, but more recent Intel/AMD CPUs have special optimizations for both NOP and MOV, executing them at the renaming stage, well before the normal execution units, so they may appear to have been executed in zero time.
For slowing down, one could use something really slow, like integer division. If that would interfere with the desired register usage, other reliable choices would be add with carry or perhaps complement carry flag. If it is not desired to modify the flags, one can use a RORX instruction for multiple bit rotation (available since Haswell, but not in older Atom CPUs), or one could execute BSWAP (available since 1989, therefore it exists in all 64-bit CPUs, including any Atom).
The NOPs in effect use up a small fraction of the instruction decode bandwidth, and if they insert enough NOPs they can reduce the number of real instructions that are issued per cycle and slow down the program with a fine degree of precision.
A double BSWAP is equivalent with a NOP, but no existing CPU does any special optimization for it and it is very unlikely that any future CPU would do something special with it, as it is mostly obsolete (nowadays MOVBE is preferable instead of it).
NOP cannot ensure a certain slow-down factor, except on exactly the same model of CPU.
The elimination will still take some amount of time, and the smaller this amount is, the better, because it allows dialing in the slowdown more precisely. Of course how many NOPs you need for a particular basic block needs to be measured on the CPU you're profiling on, but that's also the case if you use a more consistently slow instruction, since the other instructions won't necessarily maintain a consistent speed relative to it.
They might be able to skip a plain 0x90, but something like mov rax, rax would still emit a uop for the cache, before being eliminated later in rename. So at best it would be a fairly limited optimization.
It's also nice because rename is a very predictable choke point, no matter what the rest of the frontend and the backend are busy doing.
The idea is that these are unavoidably slow and preferably non-parallelizable to compute one way, but fast or near instantaneous to verify the result. Examples include the Weslowski VDFs based on similar math to RSA, MIMC, and the use of zero knowledge proofs to provide proof of slow computations.
However, like integer division, they may clobber registers that the program wants to use for other purposes.
For great slow-downs when the clobbered registers do not matter, I think that CPUID is the best, as it serializes the execution and it has a long execution time on all CPUs.
For small slow-downs I think that BSWAP is a good choice, as it modifies only 1 arbitrary register without affecting the flags, and it also is a less usual instruction so it is unlikely that it will ever receive special optimizations, like NOP and MOV.
However, multiple BSWAPs must be used, to occupy all available execution ports, otherwise if there is any execution port not occupied by the rest of the program the BSWAP may be executed concurrently, not requiring any extra time.
Later on the early PCs we had a Turbo Button, but since everyone had it in Turbo mode all the time it essentially was a way to slow down the machine.
EDIT: Found an image of what I remember as the "C64 Snail". It is called "BREMSE 64" (which is German for brake) in the image.
https://retroport.de/wp-content/uploads/2018/10/bremse64_rex...
I had the impression, the turbo button was created to slow down new PCs, so they could run old software that relied heavily on CPU speed.
With the AT it usually slowed down to some arbitrary frequency and it was more like a gimmick.
Really interesting idea. I suppose the underlying assumption is that speeding up a function might reveal a new performance floor that is much higher than the speedup would suggest. Spending time to halve a function's time only to discover that overall processing time only decreased slightly because another function is now blocking would indeed be quite bad.
Not sure how this handles I/O, or if it is enough to simply delay the results of I/O functions to simulate decreased bandwidth or increased latency.
My first idea is to taskset the code to a particular CPU core, and see if linux will let me put that core in a low frequency mode. Has anyone here done this on AMD hardware? If so, were the available frequencies slow enough to be useful for this purpose?
It’s not worth optimizing for situations that do not occur in practice.
The transistors used to detect register clearing using XOR foo,foo, on the other hand, are worth it, as lots of code has that instruction, and removing the data dependency (the instruction technically uses the contents of the foo register, but its result is independent of its value) can speed up code a lot.
However those NOPs are seldom executed frequently, because most are outside of loop bodies. Nevertheless, there are cases when NOPs may be located inside big loops, in order to align some branch targets to cache line boundaries.
That is why many recent Intel/AMD CPUs have special hardware for accelerating NOP execution, which may eliminate the NOPs before reaching the execution units.
However, there are CPUs among the Intel/AMD CPUs that can execute up to a certain number of consecutive NOPs in zero time, i.e. they are removed from the instruction stream before reaching the execution units.
In general, no instruction set architecture specifies the time needed to execute an instruction. For every specific CPU model you must search its manual to find the latency and throughput for the instruction of interest, including for NOPs.
Some CPUs, like the Intel/AMD CPUs, have multiple encodings for NOP, with different lengths in order to facilitate instruction alignment. In that case the execution time may be not the same for all kinds of NOPs.
Is this related to speculative execution? The high level description sounds like NOP works as sync points.
- Making Postgres slower (https://news.ycombinator.com/item?id=44704736)
Taking this approach seems to be effective at surfacing issues that otherwise wouldn't show up in regular testing. I could see this being useful if it was standardized to help identify issues before they hit production.
If you have a business process that is complex and owned by many different groups then causal profiling can be a good tool for dealing with the complexity. For large workflows in particular this can be powerful as the orchestration owner can experiment without much coordination (other than making sure the groups know/agree that some processing might be delayed).
https://en.wikipedia.org/wiki/Turbo_button#Purpose