c++ - PdhGetFormattedCounterValue 和正确的格式

Question

我试图理解性能 API，但我在理解PdhGetFormattedCounterValue函数和dwFormat参数时遇到了问题。

调用此函数时如何知道选择哪种格式？

我PDH_COUNTER_INFO在 MSDN 上找到了这个结构，看到这个结构有一个dwType成员，但是我仍然不明白如何使用这个结构来获取有关计数器格式的信息以成功调用该PdhGetFormattedCounterValue函数。

Answer 1

You get to choose.

Many counters are calculated as fractions or they are scaled or both, so they have fractional parts. Which makes PDH_FMT_DOUBLE a good choice.

Some counter types never have fractional parts. You could read all the documentation and work out which, and then add two code paths to handle "counters that might be fractional" and "counters that won't be fractional", but this would be a lot of work for very little gain or none.

Just use PDH_FMT_DOUBLE.

Update

For most counters the precision is only a concern during the calculation of the final value, which happens within the PDH library. But, as you say, the precision of bulk totals (such as disk free space) could be an issue. Happily, it isn't.

A double has 52 bits of significand. For a counter reported in bytes this has room for 4 petabytes without any loss of precision.

But disk free space is reported in megabytes, not bytes, so a double can report up to 2⁷² bytes or 4 zettabytes with no loss of precision. And I'm pretty sure you can't have a single volume bigger than 2⁶⁴ bytes.

Memory use (of processes and so) is reported in bytes but the values are always multiples of the page size (4096 bytes), so the bottom 12 bits of any reported value are zero. Hence a double can represent a memory size of up to 2⁶⁴ bytes - i.e. the entire 64-bit memory space - without loss of precision.

Precision isn't a problem. Just use PDH_FMT_DOUBLE.