7 ms·

Unless the programmer is programming at a very low level, the listed events occurring are out of his control. CPU caching is mostly transparent in the ISA, disk

Unless the programmer is programming at a very low level, the listed events occurring are out of his control. CPU caching is mostly transparent in the ISA, disk seeking is scheduled by the kernel or in storage controllers, file buffers are cached in the kernel, application frameworks also provide layers of caching. For the majority of programmers who want to get their shit done with straightforward code, few dependencies and acceptable performance, this is "interesting to know" but not "should know".

Iem3ohvi 8y ago
> Unless the programmer is programming at a very low level If you write unixy tools that do just one job then you often have to deal with this. For example rsync and tar put files into sequential mode and perform readaheads or writebehind-drop the page cache. And it's not just that kind of tool. At $JOB I did a fairly simple optimization to significantly reduce loadtimes (from NFS) in a render farm by importing a 3rd-party library which provided the necessary libc bindings for readaheads. It's only a dozen lines of code but reduces user-perceived latency from minutes to seconds. The PO was quite happy about not having to pay for hundreds of NVMe SSDs.
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TruffleLabs 8y ago
The visual could help programmers think about the kinds of latencies that have to work through, maybe helping highlight them in an infographic for their teams/partners.
beached_whale 8y ago
You often can control your data access patterns. Array like access, in order, is often cache friendliest
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dragontamer 8y ago
> Unless the programmer is programming at a very low level, the listed events occurring are out of his control > CPU caching is mostly transparent in the ISA Nonsense. Even in a relatively high language like Java, you can use primitive types like int[] to ensure that certain elements are close to each other in memory. As such, you can have good memory access patterns even in a high language like Java or C#. I'm fairly certain this stuff is important when choosing data-structures: Vector vs Linked List for instance. Linked Lists are harder to cache than Vectors, and this chart helps explain why both of these O(1) traversals can have dramatically different performance characteristics. > disk seeking is scheduled by the kernel or in storage controllers, file buffers are cached in the kernel But you can read a file from beginning to end. Even in a very, very high language like SQL, you can often ensure a high-speed sequential table scan if you write your joins properly. And knowledge of sequential scans can assist you in knowing which indexes to setup for your tables. Knowing that you have SSDs vs Disks can be helpful in the architecture of SQL architectures.
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- bluetomcat 8y ago
  > As such, you can have good memory access patterns even in a high language like Java or C#. CPU and data intensive heavy lifting is rarely done in such programs, it is delegated either to specialised libraries or some middleware in the form of a RDBMS. Most of these programs spend most of their time waiting for some IO event, so the few microseconds gained from the vector with a few hundred elements are negligible. > But you can read a file from beginning to end. That's what most programs actually do most of the time because files are essentially a stream abstraction. Programs that jump around a file would map it into memory, then the CPU and the kernel would do their best to cache the hot regions, even if the access to these regions is temporally or spatially distant.
  - phakding 8y ago
    > CPU and data intensive heavy lifting is rarely done in such programs How could you make such a blanket statement? is bashing java the new hipster thing to do in programming world nowadays?
sebazzz 8y ago
In general small scale Web development the only thing that matters are: 1. Are there not too many queries fired at the database (N+1) 2. Do the queries perform well Once you use SQL or any external service to your application what you do else often does not matter very much. As long as you keep the big O in the back of your mind, you would not optimize to cache lines etc.
phakding 8y ago
Not sure what very low level is, but if you are writing anything with low latency requirements you need to be cognizant of these things. As someone mentioned, using an array over a list helps CPU in memory striding. Cache misses are expensive.

Iem3ohvi 8y ago

> Unless the programmer is programming at a very low level If you write unixy tools that do just one job then you often have to deal with this. For example rsync and tar put files into sequential mode and perform readaheads or writebehind-drop the page cache. And it's not just that kind of tool. At $JOB I did a fairly simple optimization to significantly reduce loadtimes (from NFS) in a render farm by importing a 3rd-party library which provided the necessary libc bindings for readaheads. It's only a dozen lines of code but reduces user-perceived latency from minutes to seconds. The PO was quite happy about not having to pay for hundreds of NVMe SSDs.

deleted 8y ago

[deleted]

TruffleLabs 8y ago

The visual could help programmers think about the kinds of latencies that have to work through, maybe helping highlight them in an infographic for their teams/partners.

beached_whale 8y ago

You often can control your data access patterns. Array like access, in order, is often cache friendliest

deleted 8y ago

[deleted]

dragontamer 8y ago

> Unless the programmer is programming at a very low level, the listed events occurring are out of his control > CPU caching is mostly transparent in the ISA Nonsense. Even in a relatively high language like Java, you can use primitive types like int[] to ensure that certain elements are close to each other in memory. As such, you can have good memory access patterns even in a high language like Java or C#. I'm fairly certain this stuff is important when choosing data-structures: Vector vs Linked List for instance. Linked Lists are harder to cache than Vectors, and this chart helps explain why both of these O(1) traversals can have dramatically different performance characteristics. > disk seeking is scheduled by the kernel or in storage controllers, file buffers are cached in the kernel But you can read a file from beginning to end. Even in a very, very high language like SQL, you can often ensure a high-speed sequential table scan if you write your joins properly. And knowledge of sequential scans can assist you in knowing which indexes to setup for your tables. Knowing that you have SSDs vs Disks can be helpful in the architecture of SQL architectures.

deleted 8y ago
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bluetomcat 8y ago
> As such, you can have good memory access patterns even in a high language like Java or C#. CPU and data intensive heavy lifting is rarely done in such programs, it is delegated either to specialised libraries or some middleware in the form of a RDBMS. Most of these programs spend most of their time waiting for some IO event, so the few microseconds gained from the vector with a few hundred elements are negligible. > But you can read a file from beginning to end. That's what most programs actually do most of the time because files are essentially a stream abstraction. Programs that jump around a file would map it into memory, then the CPU and the kernel would do their best to cache the hot regions, even if the access to these regions is temporally or spatially distant.
- phakding 8y ago
  > CPU and data intensive heavy lifting is rarely done in such programs How could you make such a blanket statement? is bashing java the new hipster thing to do in programming world nowadays?

sebazzz 8y ago

In general small scale Web development the only thing that matters are: 1. Are there not too many queries fired at the database (N+1) 2. Do the queries perform well Once you use SQL or any external service to your application what you do else often does not matter very much. As long as you keep the big O in the back of your mind, you would not optimize to cache lines etc.

phakding 8y ago

Not sure what very low level is, but if you are writing anything with low latency requirements you need to be cognizant of these things. As someone mentioned, using an array over a list helps CPU in memory striding. Cache misses are expensive.