内存耗尽但内存仍有剩余

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最近发生了一个非常奇怪的问题。我的程序运行在一个有 8gb 内存的 docker 上，当发生恐慌时，使用了 4gb。可用内存超过 4gb，为什么会发生这种情况？下面是恐慌堆栈输出。

# ulimit -m
unlimited

# go version
1.6.2

我尝试反复运行代码行getallcombinationcomplex，并且使用的内存增长到大约5gb，但恐慌并没有再次发生。

fatal error: runtime: out of memory

runtime stack:
runtime.throw(0xd28a90, 0x16)
        /usr/local/go1.6.2/src/runtime/panic.go:547 +0x90
runtime.sysMap(0xf409e80000, 0x3ecc390000, 0x434a00, 0x1080918)
        /usr/local/go1.6.2/src/runtime/mem_linux.go:206 +0x9b
runtime.(*mheap).sysAlloc(0x1066280, 0x3ecc390000, 0xdfc4d6d680)
        /usr/local/go1.6.2/src/runtime/malloc.go:429 +0x191
runtime.(*mheap).grow(0x1066280, 0x1f661c8, 0x0)
        /usr/local/go1.6.2/src/runtime/mheap.go:651 +0x63
runtime.(*mheap).allocSpanLocked(0x1066280, 0x1f661c4, 0x10677f0)
        /usr/local/go1.6.2/src/runtime/mheap.go:553 +0x4f6
runtime.(*mheap).alloc_m(0x1066280, 0x1f661c4, 0xffffff0100000000, 0x7ff85dbfddd0)
        /usr/local/go1.6.2/src/runtime/mheap.go:437 +0x119
runtime.(*mheap).alloc.func1()
        /usr/local/go1.6.2/src/runtime/mheap.go:502 +0x41
runtime.systemstack(0x7ff85dbfdde8)
        /usr/local/go1.6.2/src/runtime/asm_amd64.s:307 +0xab
runtime.(*mheap).alloc(0x1066280, 0x1f661c4, 0x10100000000, 0x41587c)
        /usr/local/go1.6.2/src/runtime/mheap.go:503 +0x63
runtime.largeAlloc(0x3ecc386800, 0x7ff800000000, 0x6)
        /usr/local/go1.6.2/src/runtime/malloc.go:766 +0xb3
runtime.mallocgc.func3()
        /usr/local/go1.6.2/src/runtime/malloc.go:664 +0x33
runtime.systemstack(0xc820028000)
        /usr/local/go1.6.2/src/runtime/asm_amd64.s:291 +0x79
runtime.mstart()
        /usr/local/go1.6.2/src/runtime/proc.go:1051

goroutine 63798322149 [running]:
runtime.systemstack_switch()
        /usr/local/go1.6.2/src/runtime/asm_amd64.s:245 fp=0xea27f3c798 sp=0xea27f3c790
runtime.mallocgc(0x3ecc386800, 0xa572e0, 0x0, 0xec6c1e8000)
        /usr/local/go1.6.2/src/runtime/malloc.go:665 +0x9eb fp=0xea27f3c870 sp=0xea27f3c798
runtime.newarray(0xa572e0, 0x3ecc38680, 0x2)
        /usr/local/go1.6.2/src/runtime/malloc.go:798 +0xc9 fp=0xea27f3c8b0 sp=0xea27f3c870
runtime.makeslice(0xa38840, 0x3ecc38680, 0x3ecc38680, 0x0, 0x0, 0x0)
        /usr/local/go1.6.2/src/runtime/slice.go:32 +0x165 fp=0xea27f3c900 sp=0xea27f3c8b0
indexdb/db_template.XCludeList.getAllCombinationComplex(0xea4382c480, 0x8, 0x9, 0xe4ec1dc460, 0x4, 0x4, 0x0, 0x0, 0x0)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:145 +0x371 fp=0xea27f3ca88
 sp=0xea27f3c900
indexdb/db_template.XCludeList.getAllCombinationComplex(0xea4382c480, 0x8, 0x9, 0xe4ec1dc438, 0x5, 0x5, 0x0, 0x0, 0x0)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:141 +0x2ee fp=0xea27f3cc10
 sp=0xea27f3ca88
indexdb/db_template.XCludeList.getAllCombinationComplex(0xea4382c480, 0x8, 0x9, 0xe4ec1dc410, 0x6, 0x6, 0x0, 0x0, 0x0)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:141 +0x2ee fp=0xea27f3cd98
 sp=0xea27f3cc10
indexdb/db_template.XCludeList.getAllCombinationComplex(0xea4382c480, 0x8, 0x9, 0xe4ec1dc3e8, 0x7, 0x7, 0x0, 0x0, 0x0)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:141 +0x2ee fp=0xea27f3cf20
 sp=0xea27f3cd98
indexdb/db_template.XCludeList.getAllCombinationComplex(0xea4382c480, 0x8, 0x9, 0xe4ec1dc3c0, 0x8, 0x8, 0x0, 0x0, 0x0)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:141 +0x2ee fp=0xea27f3d0a8
 sp=0xea27f3cf20
indexdb/db_template.XCludeList.GetAllCombinationString(0xea4382c480, 0x8, 0x9, 0x0, 0x0, 0x0)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:114 +0x530 fp=0xea27f3d398
 sp=0xea27f3d0a8
indexdb/mem.MemDB.QueryCountersFullMatchByTags(0xead2bf0003, 0x38, 0xead2bf0043, 0x10, 0xea4382c480, 0x8, 0x9, 0x0, 0x0, 0x0, ...)
        /home/scmbuild/workspaces_cluster/index/src/indexdb/mem/mem.go:391 +0x1f5 fp=0xea27f3d670 sp=0xe
a27f3d398
indexdb/mem.(*MemDB).QueryCountersFullMatchByTags(0x107e7b8, 0xead2bf0003, 0x38, 0xead2bf0043, 0x10, 0xea4382c480, 0x8, 0x9, 0x0, 0x0, ...)
        <autogenerated>:10 +0x112 fp=0xea27f3d6d8 sp=0xea27f3d670
router.QCountersFullMatchByTags(0xd2745f5ef0)
        /home/scmbuild/workspaces_cluster/index/src/router/http.go:505 +0x54a fp=0xea27f3d8d8 sp=0xea27f
3d6d8
github.com/gin-gonic/gin.(*Context).Next(0xd2745f5ef0)
        /home/scmbuild/workspaces_cluster/index/deps/src/github.com/gin-gonic/gin/context.go:110 +0x7a f
p=0xea27f3d908 sp=0xea27f3d8d8
github.com/gin-gonic/gin.RecoveryWithWriter.func1(0xd2745f5ef0)
        /home/scmbuild/workspaces_cluster/index/deps/src/github.com/gin-gonic/gin/recovery.go:45 +0x51 f
p=0xea27f3d930 sp=0xea27f3d908
github.com/gin-gonic/gin.(*Context).Next(0xd2745f5ef0)
        /home/scmbuild/workspaces_cluster/index/deps/src/github.com/gin-gonic/gin/context.go:110 +0x7a f
p=0xea27f3d960 sp=0xea27f3d930
github.com/gin-gonic/gin.(*Engine).handleHTTPRequest(0xd4d0bd2360, 0xd2745f5ef0)
        /home/scmbuild/workspaces_cluster/index/deps/src/github.com/gin-gonic/gin/gin.go:337 +0x2fd fp=0
xea27f3dae8 sp=0xea27f3d960
github.com/gin-gonic/gin.(*Engine).ServeHTTP(0xd4d0bd2360, 0x7ff7611f00b0, 0xdb6777f1e0, 0xd6e25a0700)
        /home/scmbuild/workspaces_cluster/index/deps/src/github.com/gin-gonic/gin/gin.go:301 +0x197 fp=0
xea27f3db60 sp=0xea27f3dae8
net/http.serverHandler.ServeHTTP(0xd129858080, 0x7ff7611f00b0, 0xdb6777f1e0, 0xd6e25a0700)
        /usr/local/go1.6.2/src/net/http/server.go:2081 +0x19e fp=0xea27f3dbc0 sp=0xea27f3db60
net/http.(*conn).serve(0xce774f1400)
        /usr/local/go1.6.2/src/net/http/server.go:1472 +0xf2e fp=0xea27f3df88 sp=0xea27f3dbc0
runtime.goexit()
        /usr/local/go1.6.2/src/runtime/asm_amd64.s:1998 +0x1 fp=0xea27f3df90 sp=0xea27f3df88
created by net/http.(*Server).Serve
        /usr/local/go1.6.2/src/net/http/server.go:2137 +0x44e

解决方案

分析堆栈跟踪，看起来您正在尝试在这里分配一个太大的切片：db_template.xcludelist.getallcombinationcomplex()

indexdb/db_template.xcludelist.getallcombinationcomplex(0xea4382c480, 0x8, 0x9, 0xe4ec1dc460, 0x4, 0x4, 0x0, 0x0, 0x0)
    /home/scmbuild/workspaces_cluster/index/src/indexdb/db_template/db.go:145 +0x371 fp=0xea27f3ca88 sp=0xea27f3c900

这会调用 runtime.makeslice()：

runtime.makeslice(0xa38840, 0x3ecc38680, 0x3ecc38680, 0x0, 0x0, 0x0)
    /usr/local/go1.6.2/src/runtime/slice.go:32 +0x165 fp=0xea27f3c900 sp=0xea27f3c8b0

go 1.6 的 runtime.makeslice() 的源代码在这里：slice.go:

func makeslice(t *slicetype, len64, cap64 int64) slice {
    ...
}

它是用参数调用的：

runtime.makeslice(0xa38840, 0x3ecc38680, 0x3ecc38680, 0x0, 0x0, 0x0)

第二个值是长度，即

0x3ecc38680 = 16857138816

您尝试创建一个包含超过 16*10⁹ 元素的切片。如果切片的元素类型至少需要 1 个字节（不包括零大小的类型），则大约为 16 gb！这只是一个较低的估计。显然，在 8 gb 可用 ram 的情况下，此操作无法成功。

另外，请更新你的 go，1.6.2 已经快 3 岁了，不再受支持（甚至没有收到安全补丁）。

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