PowerShell 管道正在执行时没有垃圾收集

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问题描述

更新: PowerShell 5 似乎解决了以下错误.错误仍然存在于 3 和 4 中.因此，除非您正在运行 PowerShell 2 或5.

考虑以下代码片段:

函数 Get-DummyData() {对于 ($i = 0; $i -lt 10000000; $i++) {这太可怕了！我是忍者！更多的话，耶！"}}获取虚拟数据 |外空

这将导致 PowerShell 内存使用量无法控制地增长.执行 Get-DummyData | 后Out-Null 几次，我看到 PowerShell 内存使用量一直高达 4 GB.

根据

但是，仅仅因为您调用了 GC，并不意味着私有内存使用量将恢复到您在脚本启动之前的值.GC 收集只会收集不再使用的内存.如果有对对象的根引用，则它没有资格被收集(释放).因此，虽然 GC 系统通常不会在 C/C++ 意义上泄漏，但它们可能拥有比它们应有的更长的时间持有对象的内存囤积.

在使用内存分析器查看这一点时，似乎大部分多余的内存都被带有参数绑定信息的字符串副本占用了:

这些字符串的根如下所示:

我想知道是否有一些日志记录功能导致 PowerShell 挂在字符串化的表单管道绑定对象上?

顺便说一句，在这种特定情况下，分配给 $null 以忽略输出的内存效率要高得多:

$null = GetDummyData

另外，如果您需要简单地编辑文件，请查看 Edit-File 命令>PowerShell 社区扩展 3.2.0.只要您不使用 SingleString 开关参数，它就应该是内存高效的.

UPDATE: The following bug seems to be resolved with PowerShell 5. The bug remains in 3 and 4. So don't process any huge files with the pipeline unless you're running PowerShell 2 or 5.

Consider the following code snippet:

function Get-DummyData() {
    for ($i = 0; $i -lt 10000000; $i++) {
        "This is freaking huge!! I'm a ninja! More words, yay!"
    }
}

Get-DummyData | Out-Null

This will cause PowerShell memory usage to grow uncontrollably. After executing Get-DummyData | Out-Null a few times, I have seen PowerShell memory usage get all the way up to 4 GB.

According to ANTS Memory Profiler, we have a whole lot of things sitting around in the garbage collector's finalization queue. When I call [GC]::Collect(), the memory goes from 4 GB to a mere 70 MB. So we don't have a memory leak, strictly speaking.

Now, it's not good enough for me to be able to call [GC]::Collect() when I'm finished with a long-lived pipeline operation. I need garbage collection to happen during a pipeline operation. However if I try to invoke [GC]::Collect() while the pipeline is executing...

function Get-DummyData() {
    for ($i = 0; $i -lt 10000000; $i++) {
        "This is freaking huge!! I'm a ninja! More words, yay!"

        if ($i % 1000000 -eq 0) {
            Write-Host "Prompting a garbage collection..."
            [GC]::Collect()
        }
    }
}

Get-DummyData | Out-Null

... the problem remains. Memory usage grows uncontrollably again. I have tried several variations of this, such as adding [GC]::WaitForPendingFinalizers(), Start-Sleep -Seconds 10, etc. I have tried changing garbage collector latency modes and forcing PowerShell to use server garbage collection to no avail. I just can't get the garbage collector to do its thing while the pipeline is executing.

This isn't a problem at all in PowerShell 2.0. It's also interesting to note that $null = Get-DummyData also seems to work without memory issues. So it seems tied to the pipeline, rather than the fact that we're generating tons of strings.

How can I prevent my memory from growing uncontrollably during long pipelines?

Side note:

My Get-DummyData function is only for demonstration purposes. My real-world problem is that I'm unable to read through large files in PowerShell using Get-Content or Import-Csv. No, I'm not storing the contents of these files in variables. I'm strictly using the pipeline like I'm supposed to. Get-Content .super-huge-file.txt | Out-Null produces the same problem.

解决方案

A couple of things to point out here. First, GC calls do work in the pipeline. Here's a pipeline script that only invokes the GC:

1..10 | Foreach {[System.GC]::Collect()}

Here's the perfmon graph of GCs during the time the script ran:

However, just because you invoke the GC it doesn't mean the private memory usage will return to the value you had before your script started. A GC collect will only collect memory that is no longer used. If there is a rooted reference to an object, it is not eligible to be collected (freed). So while GC systems typically don't leak in the C/C++ sense, they can have memory hoards that hold onto objects longer than perhaps they should.

In looking at this with a memory profiler it seems the bulk of the excess memory is taken up by a copy of the string with parameter binding info:

The root for these strings look like this:

I wonder if there is some logging feature that is causing PowerShell to hang onto a string-ized form pipeline bound objects?

BTW in this specific case, it is much more memory efficient to assign to $null to ignore the output:

$null = GetDummyData

Also, if you need to simply edit a file, check out the Edit-File command in the PowerShell Community Extensions 3.2.0. It should be memory efficient as long as you don't use the SingleString switch parameter.

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