LHC@home

I have followed 3 hosts with same hardware features and configuration (around 26/04/2018).
2 of them are linux hosts which switched between native application linux and VirtualBox software.
The 3rd was running windows with VirtualBox.

Features of 3 hosts running 4-cores work units:

ID: 10391087 (linux with 72 processors)

Details | Tasks 	1 	Anonymous 	35,455.02 	12,488,165 	7.6.31 	GenuineIntel
Intel(R) Xeon(R) CPU E5-2696 v3 @ 2.30GHz [Family 6 Model 63 Stepping 2]
(72 processors) 	[2] NVIDIA GeForce GTX 1080 (4095MB) driver: 378.13 OpenCL: 1.2 	Linux
4.4.0-104-generic

Average upload rate 1112.15 KB/sec
VirtualBox VboxManage Interface (Version: 5.2.6)

ID: 10400117 (linux with 36 processors)

Details | Tasks 	16 	Anonymous 	17,054.56 	5,656,545 	7.6.31 	GenuineIntel
Intel(R) Xeon(R) CPU E5-2696 v3 @ 2.30GHz [Family 6 Model 63 Stepping 2]
(36 processors) 	NVIDIA GeForce GTX 970 (4034MB) driver: 384.11 OpenCL: 1.2 	Linux
4.4.0-108-generic

Average upload rate 542.94 KB/sec
VirtualBox VboxManage Interface (Version: 5.2.6)

ID: 10506243 (windows with 36 processors)

Details | Tasks 	22 	Anonymous 	14,435.86 	3,036,175 	7.8.3 	GenuineIntel
Intel(R) Xeon(R) CPU E5-2696 v3 @ 2.30GHz [Family 6 Model 63 Stepping 2]
(36 processors) 	NVIDIA GeForce GTX 970 (4095MB) driver: 388.31 OpenCL: 1.2 	Microsoft Windows 10
Core x64 Edition, (10.00.16299.00)

Average upload rate 418.34 KB/sec
VirtualBox COM Interface (Version: 5.1.30)

Datas recorded on the site , sorted by ascending order of efficiency:

10391087 (linux with 72 processors)

	run time	cpu time	efficiency	application
	43920,53	100111,1	0,57		vb
	28015,64	63397,32	0,57		vb
	27500,23	62383,45	0,57		vb
	42479,20	96395,74	0,57		vb
	32957,73	74989,93	0,57		vb
	26149,97	59548,46	0,57		vb
	29135,31	66353,59	0,57		vb
	26416,57	60600,05	0,57		vb
	23833,43	58114,87	0,61		vb
	24553,68	60267,45	0,61		vb
	15453,28	57939,67	0,94		native
	11367,06	43172,74	0,95		native
	15501,01	59259,57	0,96		native
	11417,63	43670,79	0,96		native
	14665,78	56135,68	0,96		native
	14286,73	55037,48	0,96		native
	19641,19	75800,60	0,96		native
	15857,75	61283,64	0,97		native
	18880,20	73188,00	0,97		native

10400117 (linux with 36 processors)

	run time	cpu time	efficiency	application
	27244,07	73584,22	0,68		vb
	27212,46	73848,47	0,68		vb
	27303,76	73899,43	0,68		vb
	27301,73	74041,98	0,68		vb
	17755,47	56687,68	0,80		vb
	17845,35	56441,30	0,79		vb
	17588,68	55602,94	0,79		vb
	17354,07	54766,45	0,79		vb
	19556,11	74582,10	0,95		native
	19419,92	75094,64	0,97		native
	19549,29	75909,89	0,97		native
	18941,23	73104,08	0,96		native
	19762,74	75918,46	0,96		native
	14899,92	57585,83	0,97		native
	15451,20	59097,37	0,96		native
	15207,48	57780,46	0,95		native

10506243 (windows with 36 processors)

	run time	cpu time	efficiency
	21305,35	79766,66	0,94
	20614,23	77333,95	0,94
	15485,98	58339,94	0,94
	15307,02	57672,48	0,94
	15457,07	58237,94	0,94
	15586,40	58834,28	0,94
	19643,13	74370,30	0,95
	15155,82	57471,86	0,95
	20232,69	76807,64	0,95
	18960,37	72407,94	0,95
	27130,88	104066,7	0,96
	21662,46	83098,69	0,96
	25821,20	99107,42	0,96
	22404,23	86214,14	0,96
	25778,69	99250,48	0,96
	25768,48	99634,19	0,97

The results are :

average cpu efficiency :

linux

id host		native	vb	Variation %
10391087	0,96	0,58	-39,58
10400117	0,96	0,73	-23,96

windows

id host			vb	
10506243		0,95

The main question is "why" ?

The difference in cpu efficiency between linux native application and virtualbox for linux host is already known , but the value is greatly different according to the number of processors included in linux host:
A reduction of 24 % for a 36 processors host and 40 % for a 72 processors.(when processors number multiplied by 2 , variation between native and virtual box multiplied by 2).
Is there an explanation ?
It sounds like processes are shared between running VMs , but normally a VM is isolated from another one...
Why do windows hosts behave differently ? (No difference in cpu efficiency between native application and windows + VirtualBox on a 36 processors)?

The hardware is the same for these 3 hosts.(Same cpu and frequency clock).(same 4-cores work units).(same virtualbox version for the 2 linux hosts).(bandwith x2 for 72 processors).
It's difficult to think that running a guest linux inside virtualbox is easier with a windows host(with many services in background) than with a linux host (where normally you can set every daemon).
Is there a parameter "forgotten" to set up for the linux guest or the linux host, to improve the situation ?
I remain perplexed.
"Maybe" virtualbox developpers should have a look into it if they really want to have a multi-platform application ?
The conclusion seems to be :
Linux hosts (with or without numerous processors ?) would rather install linux native application instead of VirtualBox (... if they want to run only ATLAS work units).(waiting for a VirtualBox improvement).