A) 90 mongos v2.2.2 connected to the cluster with 5 long connections each.
B) 1 mongos v2.2.2 connected to the cluster with hundreds of short connections concurrently.

Two different mongod crashed with the same symptom in a week, the mongod crashed
when closing many connections from B.

log1:
Thu Dec 20 16:48:14 [conn8446] end connection 123.58.167.178:51416 (345 connections now open)
Thu Dec 20 16:48:14 [conn8444] end connection 123.58.167.178:51395 (344 connections now open)
Thu Dec 20 16:48:14 [conn8450] end connection 123.58.167.178:51536 (343 connections now open)
Thu Dec 20 16:48:14 [conn8445] end connection 123.58.167.178:51403 (342 connections now open)
Thu Dec 20 16:48:14 [conn8435] end connection 123.58.167.178:51189 (341 connections now open)
Thu Dec 20 16:48:14 [conn8442] end connection 123.58.167.178:51351 (340 connections now open)
Thu Dec 20 16:48:14 [conn8447] end connection 123.58.167.178:51473 (340 connections now open)
Thu Dec 20 16:48:14 [conn8448] end connection 123.58.167.178:51498 (338 connections now open)
Thu Dec 20 16:48:14 [conn8451] end connection 123.58.167.178:51566 (337 connections now open)
Thu Dec 20 16:48:14 [conn8453] end connection 123.58.167.178:51644 (336 connections now open)
Thu Dec 20 16:48:15 [snapshotthread] cpu: elapsed:4000 writelock: 0%
Thu Dec 20 16:48:16 [conn8369] end connection 123.58.167.178:59503 (335 connections now open)
Thu Dec 20 16:48:16 [conn8412] end connection 123.58.167.178:50814 (334 connections now open)
Thu Dec 20 16:48:16 [conn8375] end connection 123.58.167.178:59531 (333 connections now open)
Thu Dec 20 16:48:16 [conn8376] end connection 123.58.167.178:59541 (332 connections now open)
Thu Dec 20 16:48:16 [conn8413] end connection 123.58.167.178:50831 (331 connections now open)
Thu Dec 20 16:48:16 [conn8378] end connection 123.58.167.178:59553 (330 connections now open)
Thu Dec 20 16:48:16 [conn8374] end connection 123.58.167.178:59530 (329 connections now open)
Thu Dec 20 16:48:16 [conn8379] end connection 123.58.167.178:59565 (328 connections now open)
Thu Dec 20 16:48:16 [conn8362] end connection 123.58.167.178:59192 (327 connections now open)
Thu Dec 20 16:48:16 [conn8363] end connection 123.58.167.178:59203 (326 connections now open)
Thu Dec 20 16:48:16 [conn8431] end connection 123.58.167.178:51125 (325 connections now open)
Thu Dec 20 16:48:16 [conn8433] end connection 123.58.167.178:51171 (324 connections now open)
Thu Dec 20 16:48:16 [conn8380] end connection 123.58.167.178:59569 (323 connections now open)
Thu Dec 20 16:48:16 [conn8416] end connection 123.58.167.178:50859 (322 connections now open)
Thu Dec 20 16:48:16 [conn8383] end connection 123.58.167.178:59577 (321 connections now open)
Thu Dec 20 16:48:16 [conn8439] end connection 123.58.167.178:51278 (320 connections now open)
Thu Dec 20 16:48:16 [conn8371] end connection 123.58.167.178:59506 (319 connections now open)
Thu Dec 20 16:48:16 [conn8430] end connection 123.58.167.178:51100 (318 connections now open)
Thu Dec 20 16:48:16 [conn8452] end connection 123.58.167.178:51637 (317 connections now open)
Thu Dec 20 16:48:16 [conn8441] end connection 123.58.167.178:51325 (316 connections now open)
Thu Dec 20 16:48:16 [conn8364] end connection 123.58.167.178:59205 (315 connections now open)
Thu Dec 20 16:48:16 [conn8373] end connection 123.58.167.178:59529 (314 connections now open)
Thu Dec 20 16:48:16 [conn8382] end connection 123.58.167.178:59575 (313 connections now open)
Thu Dec 20 16:48:16 [conn8424] end connection 123.58.167.178:50968 (312 connections now open)
Thu Dec 20 16:48:16 [conn8366] end connection 123.58.167.178:59217 (311 connections now open)
Thu Dec 20 16:48:16 [conn8428] end connection 123.58.167.178:51076 (310 connections now open)
Thu Dec 20 16:48:16 [conn8443] end connection 123.58.167.178:51364 (309 connections now open)
Thu Dec 20 16:48:16 [conn8370] end connection 123.58.167.178:59504 (308 connections now open)
Thu Dec 20 16:48:16 [conn8360] end connection 123.58.167.178:59172 (307 connections now open)
Thu Dec 20 16:48:16 [conn8377] end connection 123.58.167.178:59551 (306 connections now open)
Thu Dec 20 16:48:16 [conn8414] end connection 123.58.167.178:50833 (305 connections now open)
Thu Dec 20 16:48:16 [conn8365] end connection 123.58.167.178:59207 (304 connections now open)
Thu Dec 20 16:48:16 Invalid access at address: 0 from thread: conn8365

Thu Dec 20 16:48:16 Got signal: 11 (Segmentation fault).

Thu Dec 20 16:48:16 [conn8438] end connection 123.58.167.178:51260 (303 connections now open)
Thu Dec 20 16:48:16 [conn8415] end connection 123.58.167.178:50837 (302 connections now open)
Thu Dec 20 16:48:16 [conn8352] end connection 123.58.167.178:58905 (301 connections now open)
Thu Dec 20 16:48:16 [conn8417] end connection 123.58.167.178:50861 (300 connections now open)
Thu Dec 20 16:48:16 Backtrace:
0xaf8c41 0x5586c9 0x558c52 0x7fa0fd6d3ff0 0xb56fa8 0xb57265 0xb5727e 0x7fa0fd6caee9 0x7fa0fd6cb8d8 0x7fa0fca7eb6d
/usr/local/mongodb/bin/mongod(_ZN5mongo15printStackTraceERSo+0x21) [0xaf8c41]
/usr/local/mongodb/bin/mongod(_ZN5mongo10abruptQuitEi+0x399) [0x5586c9]
/usr/local/mongodb/bin/mongod(_ZN5mongo24abruptQuitWithAddrSignalEiP7siginfoPv+0x262) [0x558c52]
/lib/libpthread.so.0(+0xeff0) [0x7fa0fd6d3ff0]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc11ThreadCache21ReleaseToCentralCacheEPNS0_8FreeListEmi+0x2d8) [0xb56fa8]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc11ThreadCache7CleanupEv+0x45) [0xb57265]
/usr/local/mongodb/bin/mongod(ZN8tcmalloc11ThreadCache11DeleteCacheEPS0+0xe) [0xb5727e]
/lib/libpthread.so.0(+0x5ee9) [0x7fa0fd6caee9]
/lib/libpthread.so.0(+0x68d8) [0x7fa0fd6cb8d8]
/lib/libc.so.6(clone+0x6d) [0x7fa0fca7eb6d]

log2:
Fri Dec 14 20:23:02 [conn26573] end connection 123.58.167.178:33444 (313 connections now open)
Fri Dec 14 20:23:02 [conn26567] end connection 123.58.167.178:55047 (312 connections now open)
Fri Dec 14 20:23:02 [conn26570] end connection 123.58.167.178:55056 (311 connections now open)
Fri Dec 14 20:23:02 [conn26558] end connection 123.58.167.178:55018 (309 connections now open)
Fri Dec 14 20:23:02 [conn26559] end connection 123.58.167.178:55027 (307 connections now open)
Fri Dec 14 20:23:02 [conn26553] end connection 123.58.167.178:54997 (306 connections now open)
Fri Dec 14 20:23:02 [conn26549] end connection 123.58.167.178:54988 (305 connections now open)
Fri Dec 14 20:23:02 [conn26548] end connection 123.58.167.178:54979 (304 connections now open)
Fri Dec 14 20:23:02 [conn26557] end connection 123.58.167.178:55013 (307 connections now open)
Fri Dec 14 20:23:02 [conn26601] end connection 123.58.167.178:33547 (304 connections now open)
Fri Dec 14 20:23:02 [conn26538] end connection 123.58.167.178:54943 (304 connections now open)
Fri Dec 14 20:23:02 [conn26541] end connection 123.58.167.178:54956 (303 connections now open)
Fri Dec 14 20:23:02 [conn26540] end connection 123.58.167.178:54952 (302 connections now open)
Fri Dec 14 20:23:02 [conn26560] end connection 123.58.167.178:55036 (304 connections now open)
Fri Dec 14 20:23:02 [conn26537] end connection 123.58.167.178:54941 (298 connections now open)
Fri Dec 14 20:23:02 [conn26536] end connection 123.58.167.178:54935 (298 connections now open)
Fri Dec 14 20:23:02 [conn26532] end connection 123.58.167.178:54885 (297 connections now open)
Fri Dec 14 20:23:02 [conn26534] end connection 123.58.167.178:54920 (296 connections now open)
Fri Dec 14 20:23:02 [conn26543] end connection 123.58.167.178:54970 (295 connections now open)
Fri Dec 14 20:23:02 [conn26533] end connection 123.58.167.178:54912 (297 connections now open)
Fri Dec 14 20:23:02 [conn26619] end connection 123.58.167.178:33835 (293 connections now open)
Fri Dec 14 20:23:02 [conn26527] end connection 123.58.167.178:54839 (292 connections now open)
Fri Dec 14 20:23:02 [conn26531] end connection 123.58.167.178:54881 (291 connections now open)
Fri Dec 14 20:23:02 [conn26530] end connection 123.58.167.178:54874 (293 connections now open)
Fri Dec 14 20:23:02 [conn26526] end connection 123.58.167.178:54818 (288 connections now open)
Fri Dec 14 20:23:02 [conn26528] end connection 123.58.167.178:54842 (287 connections now open)
src/third_party/gperftools-2.0/src/central_freelist.cc:106] span->refcount > 0
Fri Dec 14 20:23:02 [conn26535] end connection 123.58.167.178:54933 (297 connections now open)
Fri Dec 14 20:23:02 [conn26259] end connection 123.58.167.178:58989 (286 connections now open)
Fri Dec 14 20:23:02 [conn26525] end connection 123.58.167.178:54806 (283 connections now open)
Fri Dec 14 20:23:02 [conn26529] end connection 123.58.167.178:54849 (283 connections now open)
Fri Dec 14 20:23:02 Got signal: 6 (Aborted).

Fri Dec 14 20:23:02 Backtrace:
0xaf8c41 0x5586c9 0x7faff0b48230 0x7faff0b481b5 0x7faff0b4afc0 0xb45dbb 0xb4414c 0xb443bb 0xb449cb 0xb56f6a 0xb57265 0xb5727e 0x7faff1831ee9 0x7faff18328d8 0x7faff0be5b6d
/usr/local/mongodb/bin/mongod(_ZN5mongo15printStackTraceERSo+0x21) [0xaf8c41]
/usr/local/mongodb/bin/mongod(_ZN5mongo10abruptQuitEi+0x399) [0x5586c9]
/lib/libc.so.6(+0x32230) [0x7faff0b48230]
/lib/libc.so.6(gsignal+0x35) [0x7faff0b481b5]
/lib/libc.so.6(abort+0x180) [0x7faff0b4afc0]
/usr/local/mongodb/bin/mongod(ZN8tcmalloc3LogENS_7LogModeEPKciNS_7LogItemES3_S3_S3+0x20b) [0xb45dbb]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc15CentralFreeList14ReleaseToSpansEPv+0x12c) [0xb4414c]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc15CentralFreeList18ReleaseListToSpansEPv+0x1b) [0xb443bb]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc15CentralFreeList11InsertRangeEPvS1_i+0x6b) [0xb449cb]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc11ThreadCache21ReleaseToCentralCacheEPNS0_8FreeListEmi+0x29a) [0xb56f6a]
/usr/local/mongodb/bin/mongod(_ZN8tcmalloc11ThreadCache7CleanupEv+0x45) [0xb57265]
/usr/local/mongodb/bin/mongod(ZN8tcmalloc11ThreadCache11DeleteCacheEPS0+0xe) [0xb5727e]
/lib/libpthread.so.0(+0x5ee9) [0x7faff1831ee9]
/lib/libpthread.so.0(+0x68d8) [0x7faff18328d8]
/lib/libc.so.6(clone+0x6d) [0x7faff0be5b6d]