linux free命令执行结果分析

运维同学看到一个系统的内存使用很高，但是通过其它命令却找不到相应的进程。让帮忙查看一下原因：

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# free -m               total        used        free      shared  buff/cache   available Mem:          48120       14788         509          63       32823       32963 Swap:          8191           0        8191

　　

free命令可以显示free、used物理内存，和swap内存，以及内核使用的buffers、caches。free显示的内容来自/proc/meminfo。

找了个手头的db服务器，给其分析了一下。执行不带任何参数的free命令，会显示以下类似内容：(这是低版本的linux)

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# free -m              total       used       free     shared    buffers     cached Mem:        774027     740693      33333       1442       3258     169694 -/+ buffers/cache:     567741     206286 Swap:        65535          0      65535

 

第一行：

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·total：总的内存大小。对应/proc/meminfo中的MemTotal ·used：使用的内存 ·free：未被使用的内存。对应/proc/meminfo中的MemFree ·shared：tmpfs使用的内存。对应/proc/meminfo中的Shmem ·buffers：内核buffers使用的内存。对应/proc/meminfo中的Buffers ·cached：page cache和slabs使用的内存。对应/proc/meminfo中的Cached和SReclaimable

　　

第二行：

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1	-/+ buffers/cache 的used、free值。

　　

第三行：

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1	swap内存的使用。/proc/meminfo中的SwapTotal、SwapFree

　　

以这个低版本的linux为例，如何检查真正的空闲内存是多少呢？
根据上面的used和free两列，你可能会觉得空闲内存不多了(第一行显示空闲的内存只有33333MB)。可不要被第一行迷惑了。
其实，这里总内存是774027MB，进程使用了567741MB，空闲了206286MB。
所以可用的内存是206286MB，等于(cached+buffers+free)

linux的内存，大部分内存都是用于buffers和cache。Linux总是尝试通过将可用内存用于buffers（文件系统元数据）和cache（具有文件或块设备的实际内容的页面）来加速对磁盘的操作。这有助于系统运行得更快，因为磁盘信息已经在内存中，从而节省了I/O 操作。如果程序或应用程序（如 Oracle）需要空间，Linux将释放buffers和cache，为应用程序提供内存。
如果系统运行了一段时间，你通常会在第一行的"free"字段下看到的值很小。

为了提供说服力，在redhat官网找了以下的文档。

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/5/html/tuning_and_optimizing_red_hat_enterprise_linux_for_oracle_9i_and_10g_databases/chap-oracle_9i_and_10g_tuning_guide-memory_usage_and_page_cache

 

而对于新版本linux，看到的free结果是：

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# free -m               total        used        free      shared  buff/cache   available Mem:          48120       14788         509          63       32823       32963 Swap:          8191           0        8191

新版本的linux，你可以通过available来查看真正的可用内存有多少。

 

如果想看看进程都使用了多少的内存，网上有个很好用的python脚本：

https://raw.githubusercontent.com/pixelb/ps_mem/master/ps_mem.py

 

贴一下脚本：

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#!/usr/bin/env python   # Try to determine how much RAM is currently being used per program. # Note per _program_, not per process. So for example this script # will report RAM used by all httpd process together. In detail it reports: # sum(private RAM for program processes) + sum(Shared RAM for program processes) # The shared RAM is problematic to calculate, and this script automatically # selects the most accurate method available for your kernel.   # Licence: LGPLv2 # Author:  P@draigBrady.com # Source:  https://www.pixelbeat.org/scripts/ps_mem.py   # V1.0      06 Jul 2005     Initial release # V1.1      11 Aug 2006     root permission required for accuracy # V1.2      08 Nov 2006     Add total to output #                           Use KiB,MiB,... for units rather than K,M,... # V1.3      22 Nov 2006     Ignore shared col from /proc/$pid/statm for #                           2.6 kernels up to and including 2.6.9. #                           There it represented the total file backed extent # V1.4      23 Nov 2006     Remove total from output as it's meaningless #                           (the shared values overlap with other programs). #                           Display the shared column. This extra info is #                           useful, especially as it overlaps between programs. # V1.5      26 Mar 2007     Remove redundant recursion from human() # V1.6      05 Jun 2007     Also report number of processes with a given name. #                           Patch from riccardo.murri@gmail.com # V1.7      20 Sep 2007     Use PSS from /proc/$pid/smaps if available, which #                           fixes some over-estimation and allows totalling. #                           Enumerate the PIDs directly rather than using ps, #                           which fixes the possible race between reading #                           RSS with ps, and shared memory with this program. #                           Also we can show non truncated command names. # V1.8      28 Sep 2007     More accurate matching for stats in /proc/$pid/smaps #                           as otherwise could match libraries causing a crash. #                           Patch from patrice.bouchand.fedora@gmail.com # V1.9      20 Feb 2008     Fix invalid values reported when PSS is available. #                           Reported by Andrey Borzenkov <arvidjaar@mail.ru> # V3.13     17 Sep 2018 #   https://github.com/pixelb/ps_mem/commits/master/ps_mem.py   # Notes: # # All interpreted programs where the interpreter is started # by the shell or with env, will be merged to the interpreter # (as that's what's given to exec). For e.g. all python programs # starting with "#!/usr/bin/env python" will be grouped under python. # You can change this by using the full command line but that will # have the undesirable affect of splitting up programs started with # differing parameters (for e.g. mingetty tty[1-6]). # # For 2.6 kernels up to and including 2.6.13 and later 2.4 redhat kernels # (rmap vm without smaps) it can not be accurately determined how many pages # are shared between processes in general or within a program in our case: # http://lkml.org/lkml/2005/7/6/250 # A warning is printed if overestimation is possible. # In addition for 2.6 kernels up to 2.6.9 inclusive, the shared # value in /proc/$pid/statm is the total file-backed extent of a process. # We ignore that, introducing more overestimation, again printing a warning. # Since kernel 2.6.23-rc8-mm1 PSS is available in smaps, which allows # us to calculate a more accurate value for the total RAM used by programs. # # Programs that use CLONE_VM without CLONE_THREAD are discounted by assuming # they're the only programs that have the same /proc/$PID/smaps file for # each instance.  This will fail if there are multiple real instances of a # program that then use CLONE_VM without CLONE_THREAD, or if a clone changes # its memory map while we're checksumming each /proc/$PID/smaps. # # I don't take account of memory allocated for a program # by other programs. For e.g. memory used in the X server for # a program could be determined, but is not. # # FreeBSD is supported if linprocfs is mounted at /compat/linux/proc/ # FreeBSD 8.0 supports up to a level of Linux 2.6.16   import argparse import errno import os import sys import time import io   # The following exits cleanly on Ctrl-C or EPIPE # while treating other exceptions as before. def std_exceptions(etype, value, tb):     sys.excepthook = sys.__excepthook__     if issubclass(etype, KeyboardInterrupt):         pass     elif issubclass(etype, IOError) and value.errno == errno.EPIPE:         pass     else:         sys.__excepthook__(etype, value, tb) sys.excepthook = std_exceptions   # #   Define some global variables #   PAGESIZE = os.sysconf("SC_PAGE_SIZE") / 1024 #KiB our_pid = os.getpid()   have_pss = 0 have_swap_pss = 0   class Unbuffered(io.TextIOBase):    def __init__(self, stream):        super(Unbuffered, self).__init__()        self.stream = stream    def write(self, data):        self.stream.write(data)        self.stream.flush()    def close(self):        self.stream.close()   class Proc:     def __init__(self):         uname = os.uname()         if uname[0] == "FreeBSD":             self.proc = '/compat/linux/proc'         else:             self.proc = '/proc'       def path(self, *args):         return os.path.join(self.proc, *(str(a) for a in args))       def open(self, *args):         try:             if sys.version_info < (3,):                 return open(self.path(*args))             else:                 return open(self.path(*args), errors='ignore')         except (IOError, OSError):             if type(args[0]) is not int:                 raise             val = sys.exc_info()[1]             if (val.errno == errno.ENOENT or # kernel thread or process gone                 val.errno == errno.EPERM or                 val.errno == errno.EACCES):                 raise LookupError             raise   proc = Proc()     # #   Functions #   def parse_options():     help_msg = 'Show program core memory usage.'     parser = argparse.ArgumentParser(prog='ps_mem', description=help_msg)     parser.add_argument('--version', action='version', version='3.13')     parser.add_argument(         '-s', '--split-args',         action='store_true',         help='Show and separate by, all command line arguments',     )     parser.add_argument(         '-t', '--total',         dest='only_total',         action='store_true',         help='Show only the total value',     )     parser.add_argument(         '-d', '--discriminate-by-pid',         action='store_true',         help='Show by process rather than by program',     )     parser.add_argument(         '-S', '--swap',         dest='show_swap',         action='store_true',         help='Show swap information',     )     parser.add_argument(         '-p',         dest='pids',         metavar='<pid>[,pid2,...pidN]',         help='Only show memory usage PIDs in the specified list',     )     parser.add_argument(         '-w',         dest='watch',         metavar='<N>',         type=int,         help='Measure and show process memory every N seconds',     )     args = parser.parse_args()       args.pids_to_show = []     if args.pids:         try:             args.pids_to_show = [int(x) for x in args.pids.split(',')]         except ValueError:             parser.error('Invalid PID(s): %s' % args.pids)       if args.watch is not None:         if args.watch <= 0:             parser.error('Seconds must be positive! (%s)' % args.watch)       return (         args.split_args,         args.pids_to_show,         args.watch,         args.only_total,         args.discriminate_by_pid,         args.show_swap,     )     # (major,minor,release) def kernel_ver():     kv = proc.open('sys/kernel/osrelease').readline().split(".")[:3]     last = len(kv)     if last == 2:         kv.append('0')     last -= 1     while last > 0:         for char in "-_":             kv[last] = kv[last].split(char)[0]         try:             int(kv[last])         except:             kv[last] = 0         last -= 1     return (int(kv[0]), int(kv[1]), int(kv[2]))     #return Private,Shared,Swap(Pss),unique_id #Note shared is always a subset of rss (trs is not always) def getMemStats(pid):     global have_pss     global have_swap_pss     mem_id = pid #unique     Private_lines = []     Shared_lines = []     Private_huge_lines = []     Shared_huge_lines = []     Pss_lines = []     Rss = (int(proc.open(pid, 'statm').readline().split()[1])            * PAGESIZE)     Swap_lines = []     Swap_pss_lines = []       Swap = 0       if os.path.exists(proc.path(pid, 'smaps')):  # stat         smaps = 'smaps'         if os.path.exists(proc.path(pid, 'smaps_rollup')):             smaps = 'smaps_rollup' # faster to process         lines = proc.open(pid, smaps).readlines()  # open         # Note we checksum smaps as maps is usually but         # not always different for separate processes.         mem_id = hash(''.join(lines))         for line in lines:             # {Private,Shared}_Hugetlb is not included in Pss (why?)             # so we need to account for separately.             if line.startswith("Private_Hugetlb:"):                 Private_huge_lines.append(line)             elif line.startswith("Shared_Hugetlb:"):                 Shared_huge_lines.append(line)             elif line.startswith("Shared"):                 Shared_lines.append(line)             elif line.startswith("Private"):                 Private_lines.append(line)             elif line.startswith("Pss:"):                 have_pss = 1                 Pss_lines.append(line)             elif line.startswith("Swap:"):                 Swap_lines.append(line)             elif line.startswith("SwapPss:"):                 have_swap_pss = 1                 Swap_pss_lines.append(line)         Shared = sum([int(line.split()[1]) for line in Shared_lines])         Private = sum([int(line.split()[1]) for line in Private_lines])         Shared_huge = sum([int(line.split()[1]) for line in Shared_huge_lines])         Private_huge = sum([int(line.split()[1]) for line in Private_huge_lines])         #Note Shared + Private = Rss above         #The Rss in smaps includes video card mem etc.         if have_pss:             pss_adjust = 0.5 # add 0.5KiB as this avg error due to truncation             Pss = sum([float(line.split()[1])+pss_adjust for line in Pss_lines])             Shared = Pss - Private         Private += Private_huge  # Add after as PSS doesn't a/c for huge pages         if have_swap_pss:             # The kernel supports SwapPss, that shows proportional swap share.             # Note that Swap - SwapPss is not Private Swap.             Swap = sum([int(line.split()[1]) for line in Swap_pss_lines])         else:             # Note that Swap = Private swap + Shared swap.             Swap = sum([int(line.split()[1]) for line in Swap_lines])     elif (2,6,1) <= kernel_ver() <= (2,6,9):         Shared = 0 #lots of overestimation, but what can we do?         Shared_huge = 0         Private = Rss     else:         Shared = int(proc.open(pid, 'statm').readline().split()[2])         Shared *= PAGESIZE         Shared_huge = 0         Private = Rss - Shared     return (Private, Shared, Shared_huge, Swap, mem_id)     def getCmdName(pid, split_args, discriminate_by_pid, exe_only=False):     cmdline = proc.open(pid, 'cmdline').read().split("\0")     while cmdline[-1] == '' and len(cmdline) > 1:         cmdline = cmdline[:-1]       path = proc.path(pid, 'exe')     try:         path = os.readlink(path)         # Some symlink targets were seen to contain NULs on RHEL 5 at least         # https://github.com/pixelb/scripts/pull/10, so take string up to NUL         path = path.split('\0')[0]     except OSError:         val = sys.exc_info()[1]         if (val.errno == errno.ENOENT or # either kernel thread or process gone             val.errno == errno.EPERM or             val.errno == errno.EACCES):             raise LookupError         raise       if split_args:         return ' '.join(cmdline).replace('\n', ' ')     if path.endswith(" (deleted)"):         path = path[:-10]         if os.path.exists(path):             path += " [updated]"         else:             #The path could be have prelink stuff so try cmdline             #which might have the full path present. This helped for:             #/usr/libexec/notification-area-applet.#prelink#.fX7LCT (deleted)             if os.path.exists(cmdline[0]):                 path = cmdline[0] + " [updated]"             else:                 path += " [deleted]"     exe = os.path.basename(path)     if exe_only: return exe       proc_status = proc.open(pid, 'status').readlines()     cmd = proc_status[0][6:-1]     if exe.startswith(cmd):         cmd = exe #show non truncated version         #Note because we show the non truncated name         #one can have separated programs as follows:         #584.0 KiB +   1.0 MiB =   1.6 MiB    mozilla-thunder (exe -> bash)         # 56.0 MiB +  22.2 MiB =  78.2 MiB    mozilla-thunderbird-bin     else:         #Lookup the parent's exe and use that if matching         #which will merge "Web Content" with "firefox" for example         ppid = 0         for l in range(10):             ps_line = proc_status[l]             if ps_line.startswith('PPid:'):                 ppid = int(ps_line[6:-1])                 break         if ppid:             try:                 p_exe = getCmdName(ppid, False, False, exe_only=True)             except LookupError:                 pass             else:                 if exe == p_exe:                     cmd = exe     if sys.version_info >= (3,):         cmd = cmd.encode(errors='replace').decode()     if discriminate_by_pid:         cmd = '%s [%d]' % (cmd, pid)     return cmd     #The following matches "du -h" output #see also human.py def human(num, power="Ki", units=None):     if units is None:         powers = ["Ki", "Mi", "Gi", "Ti"]         while num >= 1000: #4 digits             num /= 1024.0             power = powers[powers.index(power)+1]         return "%.1f %sB" % (num, power)     else:         return "%.f" % ((num * 1024) / units)     def cmd_with_count(cmd, count):     if count > 1:         return "%s (%u)" % (cmd, count)     else:         return cmd   #Warn of possible inaccuracies #RAM: #2 = accurate & can total #1 = accurate only considering each process in isolation #0 = some shared mem not reported #-1= all shared mem not reported #SWAP: #2 = accurate & can total #1 = accurate only considering each process in isolation #-1= not available def val_accuracy(show_swap):     """http://wiki.apache.org/spamassassin/TopSharedMemoryBug"""     kv = kernel_ver()     pid = os.getpid()     swap_accuracy = -1     if kv[:2] == (2,4):         if proc.open('meminfo').read().find("Inact_") == -1:             return 1, swap_accuracy         return 0, swap_accuracy     elif kv[:2] == (2,6):         if os.path.exists(proc.path(pid, 'smaps')):             swap_accuracy = 1             if proc.open(pid, 'smaps').read().find("Pss:")!=-1:                 return 2, swap_accuracy             else:                 return 1, swap_accuracy         if (2,6,1) <= kv <= (2,6,9):             return -1, swap_accuracy         return 0, swap_accuracy     elif kv[0] > 2 and os.path.exists(proc.path(pid, 'smaps')):         swap_accuracy = 1         if show_swap and proc.open(pid, 'smaps').read().find("SwapPss:")!=-1:             swap_accuracy = 2         return 2, swap_accuracy     else:         return 1, swap_accuracy   def show_val_accuracy( ram_inacc, swap_inacc, only_total, show_swap ):     level = ("Warning","Error")[only_total]       # Only show significant warnings     if not show_swap:         swap_inacc = 2     elif only_total:         ram_inacc = 2       if ram_inacc == -1:         sys.stderr.write(          "%s: Shared memory is not reported by this system.\n" % level         )         sys.stderr.write(          "Values reported will be too large, and totals are not reported\n"         )     elif ram_inacc == 0:         sys.stderr.write(          "%s: Shared memory is not reported accurately by this system.\n" % level         )         sys.stderr.write(          "Values reported could be too large, and totals are not reported\n"         )     elif ram_inacc == 1:         sys.stderr.write(          "%s: Shared memory is slightly over-estimated by this system\n"          "for each program, so totals are not reported.\n" % level         )       if swap_inacc == -1:         sys.stderr.write(          "%s: Swap is not reported by this system.\n" % level         )     elif swap_inacc == 1:         sys.stderr.write(          "%s: Swap is over-estimated by this system for each program,\n"          "so totals are not reported.\n" % level         )       sys.stderr.close()     if only_total:         if show_swap:             accuracy = swap_inacc         else:             accuracy = ram_inacc         if accuracy != 2:             sys.exit(1)     def get_memory_usage(pids_to_show, split_args, discriminate_by_pid,                      include_self=False, only_self=False):     cmds = {}     shareds = {}     shared_huges = {}     mem_ids = {}     count = {}     swaps = {}     for pid in os.listdir(proc.path('')):         if not pid.isdigit():             continue         pid = int(pid)           # Some filters         if only_self and pid != our_pid:             continue         if pid == our_pid and not include_self:             continue         if pids_to_show and pid not in pids_to_show:             continue           try:             cmd = getCmdName(pid, split_args, discriminate_by_pid)         except LookupError:             #operation not permitted             #kernel threads don't have exe links or             #process gone             continue           try:             private, shared, shared_huge, swap, mem_id = getMemStats(pid)         except RuntimeError:             continue #process gone         if shareds.get(cmd):             if have_pss: #add shared portion of PSS together                 shareds[cmd] += shared             elif shareds[cmd] < shared: #just take largest shared val                 shareds[cmd] = shared         else:             shareds[cmd] = shared         if shared_huges.get(cmd):             if shared_huges[cmd] < shared_huge: #just take largest shared_huge                 shared_huges[cmd] = shared_huge         else:             shared_huges[cmd] = shared_huge         cmds[cmd] = cmds.setdefault(cmd, 0) + private         if cmd in count:             count[cmd] += 1         else:             count[cmd] = 1         mem_ids.setdefault(cmd, {}).update({mem_id: None})           # Swap (overcounting for now...)         swaps[cmd] = swaps.setdefault(cmd, 0) + swap       # Total swaped mem for each program     total_swap = 0       # Add shared mem for each program     total = 0       for cmd in cmds:         cmd_count = count[cmd]         if len(mem_ids[cmd]) == 1 and cmd_count > 1:             # Assume this program is using CLONE_VM without CLONE_THREAD             # so only account for one of the processes             cmds[cmd] /= cmd_count             if have_pss:                 shareds[cmd] /= cmd_count         # overestimation possible if shared_huges shared across commands         shareds[cmd] += shared_huges[cmd]         cmds[cmd] = cmds[cmd] + shareds[cmd]         total += cmds[cmd]  # valid if PSS available         total_swap += swaps[cmd]       sorted_cmds = sorted(cmds.items(), key=lambda x:x[1])     sorted_cmds = [x for x in sorted_cmds if x[1]]       return sorted_cmds, shareds, count, total, swaps, total_swap   def print_header(show_swap, discriminate_by_pid):     output_string = " Private  +   Shared  =  RAM used"     if show_swap:         output_string += "   Swap used"     output_string += "\tProgram"     if discriminate_by_pid:         output_string += "[pid]"     output_string += "\n\n"     sys.stdout.write(output_string)     def print_memory_usage(sorted_cmds, shareds, count, total, swaps, total_swap,                        show_swap):     for cmd in sorted_cmds:           output_string = "%9s + %9s = %9s"         output_data = (human(cmd[1]-shareds[cmd[0]]),                        human(shareds[cmd[0]]), human(cmd[1]))         if show_swap:             output_string += "   %9s"             output_data += (human(swaps[cmd[0]]),)         output_string += "\t%s\n"         output_data += (cmd_with_count(cmd[0], count[cmd[0]]),)           sys.stdout.write(output_string % output_data)       # Only show totals if appropriate     if have_swap_pss and show_swap:  # kernel will have_pss         sys.stdout.write("%s\n%s%9s%s%9s\n%s\n" %                          ("-" * 45, " " * 24, human(total), " " * 3,                           human(total_swap), "=" * 45))     elif have_pss:         sys.stdout.write("%s\n%s%9s\n%s\n" %                          ("-" * 33, " " * 24, human(total), "=" * 33))     def verify_environment(pids_to_show):     if os.geteuid() != 0 and not pids_to_show:         sys.stderr.write("Sorry, root permission required, or specify pids with -p\n")         sys.stderr.close()         sys.exit(1)       try:         kernel_ver()     except (IOError, OSError):         val = sys.exc_info()[1]         if val.errno == errno.ENOENT:             sys.stderr.write(               "Couldn't access " + proc.path('') + "\n"               "Only GNU/Linux and FreeBSD (with linprocfs) are supported\n")             sys.exit(2)         else:             raise   def main():     # Force the stdout and stderr streams to be unbuffered     sys.stdout = Unbuffered(sys.stdout)     sys.stderr = Unbuffered(sys.stderr)       split_args, pids_to_show, watch, only_total, discriminate_by_pid, \     show_swap = parse_options()       verify_environment(pids_to_show)       if not only_total:         print_header(show_swap, discriminate_by_pid)       if watch is not None:         try:             sorted_cmds = True             while sorted_cmds:                 sorted_cmds, shareds, count, total, swaps, total_swap = \                     get_memory_usage(pids_to_show, split_args,                                      discriminate_by_pid)                 if only_total and show_swap and have_swap_pss:                     sys.stdout.write(human(total_swap, units=1)+'\n')                 elif only_total and not show_swap and have_pss:                     sys.stdout.write(human(total, units=1)+'\n')                 elif not only_total:                     print_memory_usage(sorted_cmds, shareds, count, total,                                        swaps, total_swap, show_swap)                   sys.stdout.flush()                 time.sleep(watch)             else:                 sys.stdout.write('Process does not exist anymore.\n')         except KeyboardInterrupt:             pass     else:         # This is the default behavior         sorted_cmds, shareds, count, total, swaps, total_swap = \             get_memory_usage(pids_to_show, split_args,                              discriminate_by_pid)         if only_total and show_swap and have_swap_pss:             sys.stdout.write(human(total_swap, units=1)+'\n')         elif only_total and not show_swap and have_pss:             sys.stdout.write(human(total, units=1)+'\n')         elif not only_total:             print_memory_usage(sorted_cmds, shareds, count, total, swaps,                                total_swap, show_swap)       # We must close explicitly, so that any EPIPE exception     # is handled by our excepthook, rather than the default     # one which is reenabled after this script finishes.     sys.stdout.close()       ram_accuracy, swap_accuracy = val_accuracy( show_swap )     show_val_accuracy( ram_accuracy, swap_accuracy, only_total, show_swap )   if __name__ == '__main__': main()