SQL Server Sessions – Hundreds of Sessions Queued in Suspended State

This isn't a complete answer, as I admit I don't know the complete answer, but can't fit into a comment.

Short detour - regarding SQLOS memory there is a wealth of information coming from a guy that worked on the internals of SQL Server storage team - Slava Oks. He has lots of articles on MSDN blogs regarding the SQLOS memory manager and they are all worth a read. Now I know we're on to SQL Server 2012, but the info is valuable nonetheless.

Now, regarding your first question, if there's a limit of max memory taken by db page cache before being cleared out and how is it controlled, I'll quote his word on it.

From article SQLOS's memory manager and SQL Server's Buffer Pool:

Buffer Pool commits pages on demand. Depending on internal memory requirements and external memory state, it calculates its target, amount of memory it thinks it should commit before it can get into memory pressure. To keep system out of paging target is constantly recalculated. Target memory can't exceed max memory that represents max server memory settings. Even if you set min server memory equal to max server memory Buffer Pool will only commit its memory on demand. You can observe this behavior by monitoring corresponding profiler event.

From this article's comment I understand that SQL Server uses a LRU algorithm (LRU = least recently used). If you really need to go in depth, SQL Server's LRU algorithm in particular is described in the following research paper: The LRU-K Page Replacement Algorithm For Database Disk Buffering.

Now, for your second question, the log caches are not really exposed so they can't be changed/tuned, as they are a part of the SQLOS internal memory structure. Quoting from an answer from MSDN forum:

Essentially each database has a log buffer/cache for its transaction log which is a small contiguous memory allocation, that is 60K in size and used by active transactions, and flushed when a transaction commits, or when the buffer space fills up. Multiple active transactions can be interleaved in the log buffer, and if any one transaction commits, the buffer memory is written to the transaction log file by the log writer. If the space fills up, the contents get flushed to disk allowing the buffer/cache memory to be reused. This is why log IO can range for 512bytes to 64K (its actually 60K), but it is always sequential so varying write sizes doesn't matter that much. The format of the log records is different than data pages as documented in the BOL (http://msdn.microsoft.com/en-us/library/ms190925.aspx), so the size of a log record is variable, but the size of the log buffer/cache is consistent.

We had to bring MS in on this one via a paid support call. After a week of dumps, xperf analysis, driver updates, playig with Virtual infrastructure we ruled out all the basics (or so we thought) - over a week later we found the problem - there was over 500,000 sys.server_event_notifications defined - these all looked as follows:

name    object_id   parent_class    parent_class_desc   parent_id   create_date modify_date service_name    broker_instance creator_sid principal_id
SQLWEP_E0B9EA66_1E73_45B7_B528_75166DC7A15D 337640  100 SERVER  0   2013-06-14 01:56:27.077 2013-06-14 01:56:27.077 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_6492D14E_99D2_49F2_9CD9_43F4B7DC7F63 338711  100 SERVER  0   2013-02-12 17:50:55.897 2013-02-12 17:50:55.897 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_C9871B51_F866_48A6_862E_08EC4F92DAFA 351457  100 SERVER  0   2013-03-06 02:04:18.070 2013-03-06 02:04:18.070 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_0E5D8FAF_1A9C_43FA_8EDF_A5713A8C7825 362061  100 SERVER  0   2014-02-08 04:57:17.217 2014-02-08 04:57:17.217 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_712ED252_E529_4022_9EA3_C785BAEB8147 364203  100 SERVER  0   2013-04-05 15:52:11.190 2013-04-05 15:52:11.190 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_E2D31BC2_E416_4D12_8B4A_88F4183FE021 374807  100 SERVER  0   2014-03-15 13:31:25.390 2014-03-15 13:31:25.390 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_7B389ACB_EA1F_434C_8B2A_7B93D4D0F098 376949  100 SERVER  0   2013-05-12 05:02:12.450 2013-05-12 05:02:12.450 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_4A828CDE_D72D_4277_97C0_030686F86377 378020  100 SERVER  0   2013-01-21 16:49:04.780 2013-01-21 16:49:04.780 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_4EA3C2AD_ACCE_4A66_83E9_0676D2250C34 387553  100 SERVER  0   2014-04-24 20:26:31.053 2014-04-24 20:26:31.053 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259
SQLWEP_6EA3C0FB_6EDA_4B9D_A3FE_D2B07799212C 389695  100 SERVER  0   2013-06-16 18:07:06.043 2013-06-16 18:07:06.043 SQL/Notifications/ProcessWMIEventProviderNotification/v1.0  706DF30B-6BD5-458B-85A4-F1C174263ADE    0x010100000000000512000000  259

These had all been created gradually over 5 years, what they are, where they come from, no one could say & MS couldn't find any related alerts to justify them either. Dropping 1 notification took 4 seconds so we would have been looking at 22 days to drop them all - suffice to say we ended up nuking the SQL instance & re-installing SQL from scratch.