在过去一周左右的时间里，我一直在调查内存使用量随时间累积的应用程序中的问题。我将其缩小到复制 a

std::vector< std::vector< std::vector< std::map< uint, map< uint, std::bitset< N> > > > > >

在工作线程中(我意识到这是一种组织内存的荒谬方式)。定期地，工作线程被销毁、重新创建，并且该线程在启动时复制该内存结构。复制的原始数据通过引用从主线程传递到工作线程。

使用 malloc_stat 和 malloc_info，我可以看到当工作线程被销毁时，它正在使用的 arena/heap 在它的 fastbins 空闲列表中保留了用于该结构的内存。这是有道理的，因为有许多小于 64 字节的单独分配。

问题是，当工作线程被重新创建时，它会创建一个新的 arena/heap 而不是重复使用之前的 arena/heap，这样来自之前 arenas/heaps 的 fastbins 永远不会被重复使用。最终，系统在重用之前的堆/竞技场以重用它们持有的 fastbins 之前耗尽内存。

有点偶然，我发现在我的主线程中调用 malloc_trim(0)，在加入工作线程后，导致线程 arenas/heaps 中的 fastbins 被释放。据我所知，这种行为没有记录。有人有解释吗？

这是我用来查看此行为的一些测试代码:

// includes
#include <stdio.h>
#include <algorithm>
#include <vector>
#include <iostream>
#include <stdexcept>
#include <stdio.h>
#include <string>
#include <mcheck.h>
#include <malloc.h>
#include <map>
#include <bitset>
#include <boost/thread.hpp>
#include <boost/shared_ptr.hpp>

// Number of bits per bitset.
const int sizeOfBitsets = 40;

// Executes a system command. Used to get output of "free -m".
std::string ExecuteSystemCommand(const char* cmd) {
    char buffer[128];
    std::string result = "";
    FILE* pipe = popen(cmd, "r");
    if (!pipe) throw std::runtime_error("popen() failed!");
    try {
        while (!feof(pipe)) {
            if (fgets(buffer, 128, pipe) != NULL)
                result += buffer;
        }
    } catch (...) {
        pclose(pipe);
        throw;
    }
    pclose(pipe);
    return result;
}

// Prints output of "free -m" and output of malloc_stat().
void PrintMemoryStats()
{
    try
    {
        char *buf;
        size_t size;
        FILE *fp;

        std::string myCommand("free -m");
        std::string result = ExecuteSystemCommand(myCommand.c_str());
        printf("Free memory is \n%s\n", result.c_str());

        malloc_stats();

        fp = open_memstream(&buf, &size);
        malloc_info(0, fp);
        fclose(fp);
        printf("# Memory Allocation Stats\n%s\n#> ", buf);
        free(buf);

    }
    catch(...)
    {
        printf("Unable to print memory stats.\n");
        throw;
    }
}

void MakeCopies(std::vector<std::vector<std::map<uint, std::map<uint, std::bitset<sizeOfBitsets> > > > >& data)
{
    try
    {
        // Create copies.
        std::vector<std::vector<std::map<uint, std::map<uint, std::bitset<sizeOfBitsets> > > > > dataCopyA(data);
        std::vector<std::vector<std::map<uint, std::map<uint, std::bitset<sizeOfBitsets> > > > > dataCopyB(data);
        std::vector<std::vector<std::map<uint, std::map<uint, std::bitset<sizeOfBitsets> > > > > dataCopyC(data);

        // Print memory info.
        printf("Memory after creating data copies:\n");
        PrintMemoryStats();
    }
    catch(...)
    {
        printf("Unable to make copies.");
        throw;
    }
}

int main(int argc, char** argv)
{
    try
    {
          // When uncommented, disables the use of fastbins.
//        mallopt(M_MXFAST, 0);

        // Print memory info.
        printf("Memory to start is:\n");
        PrintMemoryStats();

        // Sizes of original data.
        int sizeOfDataA = 2048;
        int sizeOfDataB = 4;
        int sizeOfDataC = 128;
        int sizeOfDataD = 20;
        std::vector<std::vector<std::map<uint, std::map<uint, std::bitset<sizeOfBitsets> > > > > testData;

        // Populate data.
        testData.resize(sizeOfDataA);
        for(int a = 0; a < sizeOfDataA; ++a)
        {
            testData.at(a).resize(sizeOfDataB);
            for(int b = 0; b < sizeOfDataB; ++b)
            {
                for(int c = 0; c < sizeOfDataC; ++c)
                {
                    std::map<uint, std::bitset<sizeOfBitsets> > dataMap;
                    testData.at(a).at(b).insert(std::pair<uint, std::map<uint, std::bitset<sizeOfBitsets> > >(c, dataMap));
                    for(int d = 0; d < sizeOfDataD; ++d)
                    {
                        std::bitset<sizeOfBitsets> testBitset;
                        testData.at(a).at(b).at(c).insert(std::pair<uint, std::bitset<sizeOfBitsets> >(d, testBitset));
                    }
                }
            }
        }

        // Print memory info.
        printf("Memory to after creating original data is:\n");
        PrintMemoryStats();

        // Start thread to make copies and wait to join.
        {
            boost::shared_ptr<boost::thread> makeCopiesThread = boost::shared_ptr<boost::thread>(new boost::thread(&MakeCopies, boost::ref(testData)));
            makeCopiesThread->join();
        }

        // Print memory info.
        printf("Memory to after joining thread is:\n");
        PrintMemoryStats();

        malloc_trim(0);

        // Print memory info.
        printf("Memory to after malloc_trim(0) is:\n");
        PrintMemoryStats();

        return 0;

    }
    catch(...)
    {
        // Log warning.
        printf("Unable to run application.");

        // Return failure.
        return 1;
    }

    // Return success.
    return 0;
}

malloc trim 调用前后的有趣输出是(寻找“看这里!”):

#> Memory to after joining thread is:
Free memory is
              total        used        free      shared  buff/cache   available
Mem:         257676        7361      246396          25        3918      249757
Swap:          1023           0        1023

Arena 0:
system bytes     = 1443450880
in use bytes     = 1443316976
Arena 1:
system bytes     =   35000320
in use bytes     =       6608
Total (incl. mmap):
system bytes     = 1478451200
in use bytes     = 1443323584
max mmap regions =          0
max mmap bytes   =          0
# Memory Allocation Stats
<malloc version="1">
<heap nr="0">
<sizes>
<size from="241" to="241" total="241" count="1"/>
<size from="529" to="529" total="529" count="1"/>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="2" size="770"/>
<system type="current" size="1443450880"/>
<system type="max" size="1443459072"/>
<aspace type="total" size="1443450880"/>
<aspace type="mprotect" size="1443450880"/>
</heap>
<heap nr="1">
<sizes>
<size from="33" to="48" total="48" count="1"/>
<size from="49" to="64" total="4026531712" count="62914558"/> <-- LOOK HERE!
<size from="65" to="80" total="160" count="2"/>
<size from="81" to="96" total="301989888" count="3145728"/> <-- LOOK HERE!
<size from="33" to="33" total="231" count="7"/>
<size from="49" to="49" total="1274" count="26"/>
<unsorted from="0" to="49377" total="1431600" count="6144"/>
</sizes>
<total type="fast" count="66060289" size="4328521808"/>
<total type="rest" count="6177" size="1433105"/>
<system type="current" size="4329967616"/>
<system type="max" size="4329967616"/>
<aspace type="total" size="35000320"/>
<aspace type="mprotect" size="35000320"/>
</heap>
<total type="fast" count="66060289" size="4328521808"/>
<total type="rest" count="6179" size="1433875"/>
<total type="mmap" count="0" size="0"/>
<system type="current" size="5773418496"/>
<system type="max" size="5773426688"/>
<aspace type="total" size="1478451200"/>
<aspace type="mprotect" size="1478451200"/>
</malloc>

#> Memory to after malloc_trim(0) is:
Free memory is
              total        used        free      shared  buff/cache   available
Mem:         257676        3269      250488          25        3918      253850
Swap:          1023           0        1023

Arena 0:
system bytes     = 1443319808
in use bytes     = 1443316976
Arena 1:
system bytes     =   35000320
in use bytes     =       6608
Total (incl. mmap):
system bytes     = 1478320128
in use bytes     = 1443323584
max mmap regions =          0
max mmap bytes   =          0
# Memory Allocation Stats
<malloc version="1">
<heap nr="0">
<sizes>
<size from="209" to="209" total="209" count="1"/>
<size from="529" to="529" total="529" count="1"/>
<unsorted from="0" to="49377" total="1431600" count="6144"/>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="6146" size="1432338"/>
<system type="current" size="1443459072"/>
<system type="max" size="1443459072"/>
<aspace type="total" size="1443459072"/>
<aspace type="mprotect" size="1443459072"/>
</heap>
<heap nr="1"> <---------------------------------------- LOOK HERE!
<sizes> <-- HERE!
<unsorted from="0" to="67108801" total="4296392384" count="6208"/>
</sizes>
<total type="fast" count="0" size="0"/>
<total type="rest" count="6208" size="4296392384"/>
<system type="current" size="4329967616"/>
<system type="max" size="4329967616"/>
<aspace type="total" size="35000320"/>
<aspace type="mprotect" size="35000320"/>
</heap>
<total type="fast" count="0" size="0"/>
<total type="rest" count="12354" size="4297824722"/>
<total type="mmap" count="0" size="0"/>
<system type="current" size="5773426688"/>
<system type="max" size="5773426688"/>
<aspace type="total" size="1478459392"/>
<aspace type="mprotect" size="1478459392"/>
</malloc>

#>

关于 malloc_info 输出的文档几乎没有，所以我不确定我指出的那些输出是否真的是快速 bin。为了验证它们确实是 fastbins，我取消了代码行的注释

mallopt(M_MXFAST, 0);

在加入线程后，在调用 malloc_trim(0) 之前禁用 fastbins 和堆 1 的内存使用，看起来就像在调用 malloc_trim(0) 之后启用 fastbins 一样。最重要的是，禁用 fastbins 的使用会在线程加入后立即将内存返回给系统。在启用 fastbins 的情况下加入线程后，调用 malloc_trim(0) 也会将内存返回给系统。

malloc_trim(0) 的文档指出它只能从主竞技场堆的顶部释放内存，那么这里发生了什么？我在 CentOS 7 上运行 glibc 版本 2.17。

最佳答案

malloc_trim(0) states that it can only free memory from the top of the main arena heap, so what is going on here?

它可以被称为“过时”或“不正确”的文档。 Glibc 没有 documentation of malloc_trim function ； Linux 使用手册页项目中的手册页。 malloc_trim http://man7.org/linux/man-pages/man3/malloc_trim.3.html was written in 2012 by maintainer of man-pages 的手册页是新的。可能他使用了 glibc malloc/malloc.c 源代码 http://code.metager.de/source/xref/gnu/glibc/malloc/malloc.c#675 中的一些注释

676  malloc_trim(size_t pad);
677
678  If possible, gives memory back to the system (via negative
679  arguments to sbrk) if there is unused memory at the `high' end of
680  the malloc pool. You can call this after freeing large blocks of
681  memory to potentially reduce the system-level memory requirements
682  of a program. However, it cannot guarantee to reduce memory. Under
683  some allocation patterns, some large free blocks of memory will be
684  locked between two used chunks, so they cannot be given back to
685  the system.
686
687  The `pad' argument to malloc_trim represents the amount of free
688  trailing space to leave untrimmed. If this argument is zero,
689  only the minimum amount of memory to maintain internal data
690  structures will be left (one page or less). Non-zero arguments
691  can be supplied to maintain enough trailing space to service
692  future expected allocations without having to re-obtain memory
693  from the system.
694
695  Malloc_trim returns 1 if it actually released any memory, else 0.
696  On systems that do not support "negative sbrks", it will always
697  return 0.

glibc 中的实际实现是 __malloc_trim 并且它具有迭代 arenas 的代码:

http://code.metager.de/source/xref/gnu/glibc/malloc/malloc.c#4552

4552 int
4553 __malloc_trim (size_t s)

4560  mstate ar_ptr = &main_arena;
4561  do
4562    {
4563      (void) mutex_lock (&ar_ptr->mutex);
4564      result |= mtrim (ar_ptr, s);
4565      (void) mutex_unlock (&ar_ptr->mutex);
4566
4567      ar_ptr = ar_ptr->next;
4568    }
4569  while (ar_ptr != &main_arena);

每个竞技场都使用 mtrim() (mTRIm()) 函数进行修剪，该函数调用 malloc_consolidate() 将所有空闲段从fastbins(它们不会在空闲时合并，因为它们很快)到普通空闲 block (与相邻 block 合并)

4498  /* Ensure initialization/consolidation */
4499  malloc_consolidate (av);

4111  malloc_consolidate is a specialized version of free() that tears
4112  down chunks held in fastbins. 

1581   Fastbins
1591    Chunks in fastbins keep their inuse bit set, so they cannot
1592    be consolidated with other free chunks. malloc_consolidate
1593    releases all chunks in fastbins and consolidates them with
1594    other free chunks.

The problem is, when the worker thread is recreated, it creates a new arena/heap instead of reusing the previous one, such that the fastbins from previous arenas/heaps are never reused.

这很奇怪。根据设计，glibc malloc 中的最大竞技场数受 cpu_core_count * 8(对于 64 位平台)的限制； cpu_core_count * 2(对于 32 位平台)或通过环境变量 MALLOC_ARENA_MAX/mallopt 参数 M_ARENA_MAX。

您可以限制应用程序的竞技场数量；定期调用 malloc_trim() 或以“大”大小调用 malloc()(它将调用 malloc_consolidate)，然后 free( ) 在销毁之前从您的线程获取它:

3319 _int_malloc (mstate av, size_t bytes)
3368  if ((unsigned long) (nb) <= (unsigned long) (get_max_fast ()))
 // fastbin allocation path
3405  if (in_smallbin_range (nb))
 // smallbin path; malloc_consolidate may be called
3437     If this is a large request, consolidate fastbins before continuing.
3438     While it might look excessive to kill all fastbins before
3439     even seeing if there is space available, this avoids
3440     fragmentation problems normally associated with fastbins.
3441     Also, in practice, programs tend to have runs of either small or
3442     large requests, but less often mixtures, so consolidation is not
3443     invoked all that often in most programs. And the programs that
3444     it is called frequently in otherwise tend to fragment.
3445   */
3446
3447  else
3448    {
3449      idx = largebin_index (nb);
3450      if (have_fastchunks (av))
3451        malloc_consolidate (av);
3452    }

PS: malloc_trim https://github.com/mkerrisk/man-pages/commit/a15b0e60b297e29c825b7417582a33e6ca26bf65 的手册页中有注释:

+.SH NOTES
+This function only releases memory in the main arena.
+.\" malloc/malloc.c::mTRIm():
+.\" return result | (av == &main_arena ? sYSTRIm (pad, av) : 0);

是的，有对 main_arena 的检查，但它在 malloc_trim 实现 mTRIm() 的最后，它只是为了调用 sbrk() 负偏移量。 Since 2007 (glibc 2.9 and newer) there is another method 将内存返回给操作系统:madvise(MADV_DONTNEED) 用于所有领域(glibc 补丁的作者或手册页的作者未记录)。每个领域都需要整合。还有用于修剪(munmapping)顶部 block 的 mmap 堆的代码(heap_trim/shr​​ink_heap 从慢速路径 free() 调用)，但它不是从 malloc_trim。

关于c++ - malloc_trim(0) 释放 Thread Arenas 的 Fastbins？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/40513716/