是std :: ifstream明显慢于FILE吗？ [英] Is std::ifstream significantly slower than FILE?
问题描述
我已收到通知，我的图书馆速度比应该慢，大约30+次太慢解析特定文件（文本文件，大小326 kb）。用户建议，这可能是因为我使用 std :: ifstream （大概而不是 FILE ）。 / p>
我宁愿不要盲目重写，所以我想我先检查一下，因为我的猜测将是其他地方的瓶颈。我正在读字符，所以我使用的唯一功能是 get（）， peek（） ， tellg（）/ seekg（）。
更新： >
我进行了配置，并获得了混淆输出 - gprof didn似乎认为它花了这么长时间。我重写了程序首先将整个文件读入缓冲区，它加快了大约100x。我认为问题可能是 tellg（）/ seekg（）花了很长时间，但gprof可能已经无法看到由于某种原因。在任何情况下， ifstream 不会 出现以缓冲整个文件，即使对于这个大小。
我不认为这会有所作为。特别是如果你通过char读取char，I / O的开销很可能完全支配任何 else。
为什么一次读取单个字节？
在一个326kb的文件上，最快的解决方案很可能是一次性读入内存。
std :: ifstream和C当量之间的区别基本上是一个虚函数调用或两个。如果每秒执行几千万次，它可能会有所不同，否则不会重复。文件I / O通常这么慢，以至于API用于访问它并不重要。更重要的是读/写模式。许多寻求是坏的，顺序读/写良好。
I've been informed that my library is slower than it should be, on the order of 30+ times too slow parsing a particular file (text file, size 326 kb). The user suggested that it may be that I'm using std::ifstream (presumably instead of FILE).
I'd rather not blindly rewrite, so I thought I'd check here first, since my guess would be the bottleneck is elsewhere. I'm reading character by character, so the only functions I'm using are get(), peek(), and tellg()/seekg().
Update:
I profiled, and got confusing output - gprof didn't appear to think that it took so long. I rewrote the program to read the entire file into a buffer first, and it sped up by about 100x. I think the problem may have been the tellg()/seekg() that took a long time, but gprof may have been unable to see that for some reason. In any case, ifstream does not appear to buffer the entire file, even for this size.
I don't think that'd make a difference. Especially if you're reading char by char, the overhead of I/O is likely to completely dominate anything else. Why do you read single bytes at a time? You know how extremely inefficient it is?
On a 326kb file, the fastest solution will most likely be to just read it into memory at once.
The difference between std::ifstream and the C equivalents, is basically a virtual function call or two. It may make a difference if executed a few tens of million times per second, otherwise, not reall. file I/O is generally so slow that the API used to access it doesn't really matter. What matters far more is the read/write pattern. Lots of seeks are bad, sequential reads/writes good.
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