My hope is that dropping cache would make that portion of memory unused. I assumed VM had a least recently used algorithm on ZVM paging. If 2GB of a guest memory hasn't been used for hours, that makes it a better candidate to page out by VM, rather than memory that is used by a running process or another guest where all memory is utilized.
Do I have a misconception on how that works? With the absence of CMM or equivalent, is VM natively aware that Linux cache is better to page out than used memory on another guest? And won't the Linux be excessively penalized while any paged out memory cache is paged back in? Linux uses that cache with the assumption that retrieval will be near instantaneous. I am in no way arguing with you. I am a relative VM newbie. Jon -----Original Message----- From: Linux on 390 Port [mailto:LINUX-390@VM.MARIST.EDU] On Behalf Of Mauro Souza Sent: Thursday, August 15, 2013 10:34 AM To: LINUX-390@VM.MARIST.EDU Subject: Re: linux cache Never ever think about dropping cache from Linux. You will shot your both feet. Why? 1 - zVM doesn't have any clue about what type of data is in every page. Program, data and cache pages looks just the same. Only Linux knows. 2 - Linux tries to access the cache page to see if it's clean or dirty, so it can drop the clean ones and write to disk the dirty ones. 3 - Probably zVM already saw that the page was unused and had already paged it out. 4 - zVM have to get the page back, Linux gets ejected from processing queue because of a page fault. 5 - zVM brings the page back, bring Linux back. 6 - zVM updates the "last referenced time" for that page, so it goes to the end of the "swap queue". 7 - Linux drops the clean page and forgets about it. 8 - zVM keeps unused page in memory, thinking Linux will use it. Linux just changed the status of the page to "unused" and may not use it for a long long time. 9 - Unused and forgotten page left in main memory, discarded on Linux. 10 - By the time Linux will use the page, it could be swapped out, and the page-in process have to start again. You can safely ignore the cache usage on Linux, zVM will realise the page was not in use and drop it itself. When Linux needs memory, it will reclaim cache pages automatically. Mauro http://mauro.limeiratem.com - registered Linux User: 294521 Scripture is both history, and a love letter from God. 2013/8/15 Veencamp, Jonathon D. <jdveenc...@fedins.com> > Fantastic. Thanks (again) Rob! > > -----Original Message----- > From: Linux on 390 Port [mailto:LINUX-390@VM.MARIST.EDU] On Behalf Of Rob > van der Heij > Sent: Thursday, August 15, 2013 10:17 AM > To: LINUX-390@VM.MARIST.EDU > Subject: Re: linux cache > > On 15 August 2013 16:44, Dean, David (I/S) <david_d...@bcbst.com> wrote: > > > Is there any benefit (or negative) to running a "drop_cache" > periodically? > > > > > I don't find it helpful in most cases, especially since it does not reduce > the server footprint. Shaking up memory like that may even make performance > much worse. > But I do use it in my 'cmmflush' approach where I have Linux drop the cache > and then give back that memory to z/VM. It still has some disadvantages, > but the reduction in footprint is very helpful to improve response times. > The 'periodically' would be specific moments in your workload rather than > every hour (unless maybe on development systems where I do run it from > cron). > > http://zvmperf.wordpress.com/2012/07/06/using-cmm-to-flush-a-linux-guests-memory/ > > Rob > > > ________________________________ > > The information contained in this e-mail message is intended only for the > personal and confidential use of the designated recipient(s) named above. > This message may be an attorney-client or work product communication which > is privileged and confidential. It may also contain protected health > information that is protected by federal law. If you have received this > communication in error, please notify us immediately by telephone and > destroy (shred) the original message and all attachments. 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This message may be an attorney-client or work product communication which is privileged and confidential. It may also contain protected health information that is protected by federal law. If you have received this communication in error, please notify us immediately by telephone and destroy (shred) the original message and all attachments. Any review, dissemination, distribution or copying of this message by any person other than the intended recipient(s) or their authorized agents is strictly prohibited. Thank you. ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
