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在過去的一週左右，我一直在調查內存使用量隨着時間累積的應用程序中的問題。我把範圍縮小到一個行拷貝malloc_trim（0）釋放線程競技場的倉位？
std::vector< std::vector< std::vector< std::map< uint, map< uint, std::bitset< N> > > > > >
（我知道這是組織內存荒謬的方式）。定期，工作線程被銷燬，重新創建，並且該線程在啓動時複製該內存結構。被複制的原始數據通過主線程的引用傳遞給工作線程。
使用malloc_stat和malloc_info，我可以看到，當工作線程被銷燬時，它所使用的競技場/堆將在其空閒空閒列表中保留用於該結構的內存。這是有道理的，因爲有很多單獨的分配小於64字節。
問題是，當工作線程被重新創建時，它會創建一個新的競技場/堆，而不是重複使用前一個，這樣以前的場/堆的空位將不會被重用。最終，系統在重用先前的堆/競技場重用他們所持有的齋戒之前耗盡內存。
有點意外，我發現在加入工作線程後，在我的主線程中調用malloc_trim（0）會導致線程arenas/heap中的fastbins被釋放。就我所見，這種行爲是無證的。有沒有人有解釋？
這裏是我使用的是看這個行爲的一些測試代碼：
// 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;
}
從前後修整的malloc調用（尋找「看這裏！」）有趣的輸出：
#> 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>
#>
malloc_info的輸出很少或沒有文檔，所以我不確定那些我指出的輸出是否真的很快。爲了驗證他們確實是fastbins，我取消註釋代碼行
mallopt(M_MXFAST, 0);
禁止使用fastbins和加入線程後堆1的內存使用情況，要求malloc_trim（0），貌似在做之前在調用malloc_trim（0）後啓用fastbins。最重要的是，禁止使用fastbins會在線程加入後立即將內存返回給系統。調用malloc_trim（0）後，加入啓用了fastbins的線程後，還會將內存返回給系統。
malloc_trim（0）的文檔聲明它只能從主存儲區堆的頂部釋放內存，所以這裏是怎麼回事？我正在使用glibc 2.17版的CentOS 7上運行。