(This post is regarding High Frequency type programming)
I recently saw on a forum (I think they were discussing Java) that if you have to parse a lot of string data its better to use a byte array than a string with a split(). The exact post was:
One performance trick to working with any language, C++, Java, C# is to avoid object creation. It's not the cost of allocation or GC, its the cost to access large memory arrays that dont fit in the CPU cache.
Modern CPU's are much faster than their memory. They stall for many, many cycles for each cache miss. Most of the CPU transister budget is allocated to reduce this with large caches and lots of ticks.
GPU's solve the problem differently by having lots of threads ready to execute to hide memory access latency and have little or no cache and spend the transistors on more cores.
So, for example, rather than using String's and split to parse a message, use byte arrays that can be updated in place. You really want to avoid random memory access over large data structures, at least in the inner loops.
Is he just saying "dont use strings because they're an object and creating objects is costly" ? Or is he saying something else?
Does using a byte array ensure the data remains in the cache for as long as possible? When you use a string is it too large to be held in the CPU cache? Generally, is using the primitive data types the best methods for writing faster code?
Since bytes is the binary data while String is character data. It is important to know the original encoding of the text from which the byte array has created. When we use a different character encoding, we do not get the original string back.
A String is stored as an array of Unicode characters in Java. To convert it to a byte array, we translate the sequence of characters into a sequence of bytes. For this translation, we use an instance of Charset. This class specifies a mapping between a sequence of chars and a sequence of bytes.
The crucial thing about a byte array is that it gives indexed (fast), precise, raw access to each 8-bit value being stored in that part of memory, and you can operate on those bytes to control every single bit.
Byte and char arrays are often used in Java to temporarily store data internally in an application. As such arrays are also a common source or destination of data. You may also prefer to load a file into an array, if you need to access the contents of that file a lot while the program is running.
He's saying that if you break a chunk text up into separate string objects, those string objects have worse locality than the large array of text. Each string, and the array of characters it contains, is going to be somewhere else in memory; they can be spread all over the place. It is likely that the memory cache will have to thrash in and out to access the various strings as you process the data. In contrast, the one large array has the best possible locality, as all the data is on one area of memory, and cache-thrashing will be kept to a minimum.
There are limits to this, of course: if the text is very, very large, and you only need to parse out part of it, then those few small strings might fit better in the cache than the large chunk of text.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With