CPU always on 15% on idle server

[Esteban Feldman]

Hi all. I have a test instance of mongodb on a VM and is using 15% CPU all time even with no connections, here is my server status
> db.serverStatus()
{
        "host" : "virtual-1",
        "version" : "1.8.2",
        "process" : "mongod",
        "uptime" : 2966,
        "uptimeEstimate" : 2221,
        "localTime" : ISODate("2011-06-30T22:47:11.318Z"),
        "globalLock" : {
                "totalTime" : 2965931789,
                "lockTime" : 134725,
                "ratio" : 0.00004542417344177162,
                "currentQueue" : {
                        "total" : 0,
                        "readers" : 0,
                        "writers" : 0
                },
                "activeClients" : {
                        "total" : 0,
                        "readers" : 0,
                        "writers" : 0
                }
        },
        "mem" : {
                "bits" : 64,
                "resident" : 1,
                "virtual" : 545,
                "supported" : true,
                "mapped" : 464
        },
        },
        "connections" : {
                "current" : 1,
                "available" : 15999
        },
        "extra_info" : {
                "note" : "fields vary by platform",
                "heap_usage_bytes" : 292752,
                "page_faults" : 99
        },
        "indexCounters" : {
                "btree" : {
                        "accesses" : 0,
                        "hits" : 0,
                        "misses" : 0,
                        "resets" : 0,
                        "missRatio" : 0
                }
        },
        "backgroundFlushing" : {
                "flushes" : 49,
                "total_ms" : 2676,
                "average_ms" : 54.61224489795919,
                "last_ms" : 1039,
                "last_finished" : ISODate("2011-06-30T22:46:46.635Z")
        },
        "cursors" : {
                "totalOpen" : 0,
                "clientCursors_size" : 0,
                "timedOut" : 0
        },
        "network" : {
                "bytesIn" : 1660,
                "bytesOut" : 18200,
                "numRequests" : 25
        },
        "opcounters" : {
                "insert" : 0,
                "query" : 2,
                "update" : 0,
                "delete" : 0,
                "getmore" : 0,
                "command" : 25
        },
        "asserts" : {
                "regular" : 0,
                "warning" : 0,
                "msg" : 0,
                "user" : 0,
                "rollovers" : 0
        },
        "writeBacksQueued" : false,
        "ok" : 1
}

[Esteban Feldman]

Sorry... I meant to ask if that is a normal behavior or if there is something wrong...

cheers

[Nat]

which VM do you use? How much CPU usage on the host? mongodb has some threads wake up regularly to perform jobs so VM may not allocate timeslice effectively.

[Esteban Feldman]

Is a VirtualBox VM that I set up for testing MongoDB. Is sitting idle doing nothing but MongoDB.
htop reads 15% for CPU on 1 of mongodb processes always the same PID. The whole CPU is at 3% - 8% varying.

[Esteban Feldman]

I ran an strace on mongodb pid and is doing

select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout)

when I do that to other processes like cheroke they are on wait()

[Esteban Feldman]

A better output will be:

00:34:52.075162 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.011655>
00:34:52.086935 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.013691>
00:34:52.100810 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.017073>
00:34:52.118109 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.013121>
00:34:52.131458 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.013683>
00:34:52.145345 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.011541>
00:34:52.157000 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.012111>
00:34:52.169335 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.017613>
00:34:52.187230 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.013577>
00:34:52.201143 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.017174>
00:34:52.218483 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.011901>
00:34:52.230518 select(8, [6 7], NULL, NULL, {0, 10000}) = 0 (Timeout) <0.011701>

[Nat]

Ok. VirtualBox has another bug that uses CPU more than necessary. If you search on virtualbox bug database, you will find a couple of them. If that's the case, even when you pause your vm, you will still see some cpu usage on your vm process.

---

From: Esteban Feldman <ekagaur...@gmail.com>

Sender: mongod...@googlegroups.com

Date: Thu, 30 Jun 2011 17:24:28 -0700 (PDT)

To: <mongod...@googlegroups.com>

ReplyTo: mongod...@googlegroups.com

Subject: [mongodb-user] Re: CPU always on 15% on idle server

Is a VirtualBox VM that I set up for testing MongoDB. Is sitting idle doing nothing but MongoDB.
htop reads 15% for CPU on 1 of mongodb processes always the same PID. The whole CPU is at 3% - 8% varying.

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[Esteban Feldman]

Nat: I understand that, I'm not measuring VM's CPU usage, I'm measuring Mongo's inside the VM, in that same VM I have cherokee and postgresql with 0% CPU usage cause they are idle as well. But if you say it is a VM bug I will believe you.

Thanks

[Eliot Horowitz]

Its normal for mongo to use some cpu.
See: https://jira.mongodb.org/browse/SERVER-2114

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