I'll have to check the code, but I think it's possible for ram cache to use other sizes, if the objects are small. The cache knows the approximate size of the object before reading and IIRC gets an iobuf of roughly that size for the disk read. If the object goes in ram cache, the iobuf is simply moved over to the ram cache.
On Wednesday, May 24, 2017, 4:28:46 PM CDT, Sudheer Vinukonda <[email protected]> wrote:AFAIR, I don't think RAM Cache includes all ioBuf pools. It's typically the 1M (or may be higher) pools only. The rest of the lower sized ioBuf pools (especially, ioBuf-0 that's referenced here) are generally used for various purposes such as session contexts, connection objects etc. And I don't believe these pools are directly constrained or affected by RAM cache size parameter. They are simply proportional to the peak amount of concurrent sessions that ATS handles. Disclaimer : It's possible I may not be remembering correctly and it's been a while I looked at this. - Sudheer On May 24, 2017, at 12:50 PM, Kapil Sharma (kapsharm) <[email protected]> wrote: On May 24, 2017, at 2:52 PM, Dunkin, Nick <[email protected]> wrote: Hi again, This is great stuff, but it leads me to believe that I’ve totally overestimated my ram_cache.size setting. And in fact, totally misunderstood the parameter. Let me see if I understand what you’ve explained: If I expect 5 of my ioBufAllocators to be in use during normal activity, then potentially I could see memory allocated to the level of (5 x ram_cache.size)? Because each ioBufAllocator is bounded by ram_cache.size? No, not really. I guess my example which was intended as worst case example may have confused things :)Let's differentiate between:“Allocated" Chunks: IO Buffer chunks that have been allocated by the ioBufAllocator, but not all of them are actually being used in RAM Cache."In-use” Chunks: Chunks that are in use in the RAM Cache, these are subset of the “allocated” chunks.“Free” Chunks: These contain the difference between the above two. When ioBufAllocator needs chunks of a particular size pool, it will try get it from the free list. If not available, only then new chunks are allocated from memory.Allocated Chunks = In-use Chunks + Free Chunks When buffer chunk is “de-alloced” from RAM cache, it is put back into the free chunk pool. Ram Cache size parameter will limit the total “in-use” chunks, and this includes sum total size of the “in-use" chunks from all 15 pools. In general your traffic pattern should fall into steady state “plateau” such that all the “allocated” chunks doesn’t need to grow. But yes, sum total size of allocated chunks >= Ram cache size parameter. So it is best to keep some headroom in RAM. I remember there was a way to dump the mem pools information to traffic.out - maybe someone in the list can help. Hope this doesn’t confuse things more :) (Terminology I used above may not reflect what’s in the code) In which case I need to reduce, or tune, my ram_cache.size by a factor of 5? I have a large ram_cache.size (100gb), assuming it was allocated to one large reserve of memory, so I assume this understanding is naive? Thanks again for all your assistance, Nick From: "Kapil Sharma (kapsharm)" <[email protected]> Reply-To: "[email protected]" <[email protected]> Date: Wednesday, May 24, 2017 at 11:29 AM To: "[email protected]" <[email protected]> Subject: Re: Understanding ioBufAllocator behvaiour On plateauing - not necessarily; we do see the memory consumption increasing continuously in our deployments as well. It depends on the pattern of segment sizes over time. ATS uses power of 2 allocators for memory pool - there are 15 of those, ranging from 128bytes to 2M if my memory serves me right - and these are per thread! ATS will choose an optimal allocator for the segments. As Alan mentioned, once chunk are allocated, they are never freed. Here is a totally artificial example just to make the point (please correct if my understanding is flawed): * the traffic pattern was such that initially only 2M allocators were used then ATS will keep allocating 2M chunks until RAM cache limit (lets say it is 64GB) is reached. * Now traffic pattern changed (smaller fragment requests), and only 1M allocators are used, ATS will now keep allocating 1M chunks, again capping at 64GB. But in the end ATS would have allocated 128GB well over RAM cache size limit…. In the past a there was some prototype of reclaimable buffer support added in ATS, but I believe it was removed in 7.0? Also there is recent discussion of adding jmalloc? On May 24, 2017, at 11:01 AM, Alan Carroll <[email protected]> wrote: One issue is that memory never moves between the iobuf sizes. Once a chunk of memory is used for a specific iobuf slot, it's there forever. But unless something is leaking, the total size should eventually plateau, certainly within less than a day if you have a basically constant load. There will be some growth due to blocks being kept in thread local allocation pools, but again that should level in less time than you've run. On Wednesday, May 24, 2017, 9:50:39 AM CDT, Dunkin, Nick <[email protected]> wrote: Hi Alan, This is 7.0.0 I only see this behavior on ioBufAllocator[0], [4] and [5]. The other ioBufAllocators’ usage looks as I would expect (i.e. allocated goes up then flat), so I was thinking it was more likely something to do with my configuration or use-case. I’d also just like to understand, at a high level, how the ioBufAllocators are used. Thanks, Nick From: Alan Carroll <[email protected]> Reply-To: "[email protected]" <[email protected]> Date: Wednesday, May 24, 2017 at 10:33 AM To: "[email protected]" <[email protected]> Subject: Re: Understanding ioBufAllocator behvaiour Honestly it sounds like a leak. Can you specify which version of Traffic Server this is? On Wednesday, May 24, 2017, 8:22:46 AM CDT, Dunkin, Nick <[email protected]> wrote: Hi I have a load test that I’ve been running for a number of days now. I’m using the memory dump logging in traffic.out and I’m trying to understand how Traffic Server allocates and reuses memory. I’m still quite new to Traffic Server. Nearly all of the memory traces look as I would expect, i.e. memory is allocated and reused over the lifetime of the test. However my readings from ioBufAllocator[0] show a continual increase in allocated AND used. I am attaching a graph. (FYI – This graph covers approximately 3 days of continual load test.) I would have expected to start seeing reuse in ioBufAllocator by now, like I do in the other ioBufAllocators. Can someone help me understand what I’m seeing? Many thanks, Nick Dunkin Nick Dunkin Principal Engineer o: 678.258.4071 e: [email protected] 4375 River Green Pkwy # 100, Duluth, GA 30096, USA <image001.png> <image001.png>
