大对象堆分片 [英] Large Object Heap Fragmentation

问题描述

我正在处理的 C#/.NET 应用程序正在遭受缓慢的内存泄漏.我已经将 CDB 与 SOS 一起使用来尝试确定发生了什么，但数据似乎没有任何意义，所以我希望你们中的一个人以前可能经历过这种情况.

The C#/.NET application I am working on is suffering from a slow memory leak. I have used CDB with SOS to try to determine what is happening but the data does not seem to make any sense so I was hoping one of you may have experienced this before.

应用程序在 64 位框架上运行.它不断地计算数据并将其序列化到远程主机，并且对大对象堆 (LOH) 产生了相当大的影响.但是，我希望大多数 LOH 对象都是暂时的:一旦计算完成并已发送到远程主机，就应该释放内存.然而，我看到的是大量(实时)对象数组与空闲内存块交错，例如，从 LOH 中获取一个随机段:

The application is running on the 64 bit framework. It is continuously calculating and serialising data to a remote host and is hitting the Large Object Heap (LOH) a fair bit. However, most of the LOH objects I expect to be transient: once the calculation is complete and has been sent to the remote host, the memory should be freed. What I am seeing, however, is a large number of (live) object arrays interleaved with free blocks of memory, e.g., taking a random segment from the LOH:

0:000> !DumpHeap 000000005b5b1000  000000006351da10
         Address               MT     Size
...
000000005d4f92e0 0000064280c7c970 16147872
000000005e45f880 00000000001661d0  1901752 Free
000000005e62fd38 00000642788d8ba8     1056       <--
000000005e630158 00000000001661d0  5988848 Free
000000005ebe6348 00000642788d8ba8     1056
000000005ebe6768 00000000001661d0  6481336 Free
000000005f214d20 00000642788d8ba8     1056
000000005f215140 00000000001661d0  7346016 Free
000000005f9168a0 00000642788d8ba8     1056
000000005f916cc0 00000000001661d0  7611648 Free
00000000600591c0 00000642788d8ba8     1056
00000000600595e0 00000000001661d0   264808 Free
...

显然，如果我的应用程序在每次计算期间都创建长期存在的大型对象，我会期望这种情况.(它确实这样做，我接受会有一定程度的 LOH 碎片，但这不是这里的问题.)问题是您可以在上面的转储中看到的非常小的(1056 字节)对象数组，而我在代码中看不到正在创建并且以某种方式保持根植.

Obviously I would expect this to be the case if my application were creating long-lived, large objects during each calculation. (It does do this and I accept there will be a degree of LOH fragmentation but that is not the problem here.) The problem is the very small (1056 byte) object arrays you can see in the above dump which I cannot see in code being created and which are remaining rooted somehow.

还要注意，当堆段被转储时，CDB 没有报告类型:我不确定这是否相关.如果我转储标记的 (<--) 对象，CDB/SOS 会报告它正常:

Also note that CDB is not reporting the type when the heap segment is dumped: I am not sure if this is related or not. If I dump the marked (<--) object, CDB/SOS reports it fine:

0:015> !DumpObj 000000005e62fd38
Name: System.Object[]
MethodTable: 00000642788d8ba8
EEClass: 00000642789d7660
Size: 1056(0x420) bytes
Array: Rank 1, Number of elements 128, Type CLASS
Element Type: System.Object
Fields:
None

对象数组的元素都是字符串，这些字符串可以从我们的应用程序代码中识别出来.

The elements of the object array are all strings and the strings are recognisable as from our application code.

此外，我无法找到它们的 GC 根，因为 !GCRoot 命令挂起并且永远不会回来(我什至尝试将其放置一夜).

Also, I am unable to find their GC roots as the !GCRoot command hangs and never comes back (I have even tried leaving it overnight).

因此，如果有人能够解释为什么这些小(<85k)对象数组最终会出现在 LOH 中，我将非常感激:.NET 在什么情况下会在其中放置一个小对象数组?另外，有没有人碰巧知道另一种确定这些对象的根源的方法?

So, I would very much appreciate it if anyone could shed any light as to why these small (<85k) object arrays are ending up on the LOH: what situations will .NET put a small object array in there? Also, does anyone happen to know of an alternative way of ascertaining the roots of these objects?

更新 1

我昨天晚些时候提出的另一个理论是，这些对象数组开始时很大，但已经缩小，留下了在内存转储中很明显的可用内存块.让我怀疑的是对象数组总是看起来是 1056 字节长(128 个元素)，128 * 8 用于引用和 32 字节的开销.

Another theory I came up with late yesterday is that these object arrays started out large but have been shrunk leaving the blocks of free memory that are evident in the memory dumps. What makes me suspicious is that the object arrays always appear to be 1056 bytes long (128 elements), 128 * 8 for the references and 32 bytes of overhead.

这个想法可能是库或 CLR 中的某些不安全代码破坏了数组头中的元素数量字段.我知道有点远……

The idea is that perhaps some unsafe code in a library or in the CLR is corrupting the number of elements field in the array header. Bit of a long shot I know...

更新 2

感谢 Brian Rasmussen(请参阅已接受的答案)，问题已被确定为字符串实习表引起的 LOH 碎片！我写了一个快速测试应用程序来确认这一点:

Thanks to Brian Rasmussen (see accepted answer) the problem has been identified as fragmentation of the LOH caused by the string intern table! I wrote a quick test application to confirm this:

static void Main()
{
    const int ITERATIONS = 100000;

    for (int index = 0; index < ITERATIONS; ++index)
    {
        string str = "NonInterned" + index;
        Console.Out.WriteLine(str);
    }

    Console.Out.WriteLine("Continue.");
    Console.In.ReadLine();

    for (int index = 0; index < ITERATIONS; ++index)
    {
        string str = string.Intern("Interned" + index);
        Console.Out.WriteLine(str);
    }

    Console.Out.WriteLine("Continue?");
    Console.In.ReadLine();
}

应用程序首先在循环中创建和取消引用唯一的字符串.这只是为了证明在这种情况下内存不会泄漏.显然它不应该也不应该.

The application first creates and dereferences unique strings in a loop. This is just to prove that the memory does not leak in this scenario. Obviously it should not and it does not.

在第二个循环中，创建并保留了唯一的字符串.此操作将它们植根于实习表中.我没有意识到实习表是如何表示的.看起来它由一组页面组成——128 个字符串元素的对象数组——在 LOH 中创建.这在 CDB/SOS 中更为明显:

In the second loop, unique strings are created and interned. This action roots them in the intern table. What I did not realise is how the intern table is represented. It appears it consists of a set of pages -- object arrays of 128 string elements -- that are created in the LOH. This is more evident in CDB/SOS:

0:000> .loadby sos mscorwks
0:000> !EEHeap -gc
Number of GC Heaps: 1
generation 0 starts at 0x00f7a9b0
generation 1 starts at 0x00e79c3c
generation 2 starts at 0x00b21000
ephemeral segment allocation context: none
 segment    begin allocated     size
00b20000 00b21000  010029bc 0x004e19bc(5118396)
Large object heap starts at 0x01b21000
 segment    begin allocated     size
01b20000 01b21000  01b8ade0 0x00069de0(433632)
Total Size  0x54b79c(5552028)
------------------------------
GC Heap Size  0x54b79c(5552028)

转储 LOH 段揭示了我在泄漏应用程序中看到的模式:

Taking a dump of the LOH segment reveals the pattern I saw in the leaking application:

0:000> !DumpHeap 01b21000 01b8ade0
...
01b8a120 793040bc      528
01b8a330 00175e88       16 Free
01b8a340 793040bc      528
01b8a550 00175e88       16 Free
01b8a560 793040bc      528
01b8a770 00175e88       16 Free
01b8a780 793040bc      528
01b8a990 00175e88       16 Free
01b8a9a0 793040bc      528
01b8abb0 00175e88       16 Free
01b8abc0 793040bc      528
01b8add0 00175e88       16 Free    total 1568 objects
Statistics:
      MT    Count    TotalSize Class Name
00175e88      784        12544      Free
793040bc      784       421088 System.Object[]
Total 1568 objects

请注意，对象数组大小为 528(而不是 1056)，因为我的工作站是 32 位，而应用程序服务器是 64 位.对象数组仍然有 128 个元素.

Note that the object array size is 528 (rather than 1056) because my workstation is 32 bit and the application server is 64 bit. The object arrays are still 128 elements long.

所以这个故事的寓意是要非常小心地实习.如果您正在实习的字符串不知道是有限集的成员，那么您的应用程序将由于 LOH 的碎片而泄漏，至少在 CLR 的第 2 版中是这样.

So the moral to this story is to be very careful interning. If the string you are interning is not known to be a member of a finite set then your application will leak due to fragmentation of the LOH, at least in version 2 of the CLR.

在我们的应用程序中，反序列化代码路径中有通用代码，在解组期间实习实体标识符:我现在强烈怀疑这是罪魁祸首.然而，开发者的意图显然是好的，因为他们想确保如果同一个实体被多次反序列化，那么只有一个标识符字符串的实例会保留在内存中.

In our application's case, there is general code in the deserialisation code path that interns entity identifiers during unmarshalling: I now strongly suspect this is the culprit. However, the developer's intentions were obviously good as they wanted to make sure that if the same entity is deserialised multiple times then only one instance of the identifier string will be maintained in memory.

推荐答案

CLR 使用 LOH 来预分配一些对象(例如 用于内部字符串的数组).其中一些小于 85000 字节，因此通常不会在 LOH 上分配.

The CLR uses the LOH to preallocate a few objects (such as the array used for interned strings). Some of these are less than 85000 bytes and thus would not normally be allocated on the LOH.

这是一个实现细节，但我认为这样做的原因是为了避免不必要的实例垃圾收集，这些实例应该在进程自身期间存活.

It is an implementation detail, but I assume the reason for this is to avoid unnecessary garbage collection of instances that are supposed to survive as long as the process it self.

同样由于一些深奥的优化，1000 个或更多元素的任何 double[] 也会在 LOH 上分配.

Also due to a somewhat esoteric optimization, any double[] of 1000 or more elements is also allocated on the LOH.

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