Hi Mark,
Wow that's great information, thank you for the details. I hate to press the point though, but I still don't understand how the two numbers for osd_memory_target and bluestore_cache_size are actually used. Since I have disabled autotuning, is the osd_memory_target used for the onode and rocksdb cache while bluestore_cache_size is used for bluestore buffers? My cluster is only used for RBD storage, which one is more important?
Josh, to answer your questions from earlier, I've read through the cache sizing page and the documentation for the options at least a dozen times and I remain confused. Am I understanding you correctly, that all of the bluestore cache is contained within the amount set by osd_memory_target? This simply does not match my experience -- even right now with bluestore_cache_size=10Gi and osd_memory_target=6Gi, each daemon is using between 15-20 GiB. I previously set them both to 8 GiB and the memory usage was about 18 GiB per daemon. When autotuning was enabled (in Nautilus) each daemon would use about 4 GiB no matter how the values were set (and I've never found any settings to keep daemons in our lab environment from using at least 2 GiB). This cluster uses NVMe drives for storage, currently running one daemon per drive. We're planning to split them into multiple daemons per drive (following popular online wisdom) but I'm concerned about running out of RAM if I can't control how much memory each daemon will use. Let's say osd_memory_target=8Gi and bluestore_cache_size=6Gi... what are the remaining 2Gi used for? Is there a way I can calculate an appropriate amount (or even minimum amount), perhaps based on number of pools/PGs/OSDs/etc? We've had to clean up damage caused by OSD daemons getting OOMkilled before, that was a bad week.
When I dump the mempools for one OSD daemon where "ps" currently shows RSS 17235900 and VSZ 18584008:
# ceph tell osd.0 dump_mempools
{
"mempool": {
"by_pool": {
"bloom_filter": {
"items": 0,
"bytes": 0
},
"bluestore_alloc": {
"items": 25328063,
"bytes": 263022424
},
"bluestore_cache_data": {
"items": 2017,
"bytes": 57925872
},
"bluestore_cache_onode": {
"items": 400268,
"bytes": 246565088
},
"bluestore_cache_meta": {
"items": 58839368,
"bytes": 643004516
},
"bluestore_cache_other": {
"items": 63930648,
"bytes": 2770468348
},
"bluestore_Buffer": {
"items": 20,
"bytes": 1920
},
"bluestore_Extent": {
"items": 17760199,
"bytes": 852489552
},
"bluestore_Blob": {
"items": 17567934,
"bytes": 1827065136
},
"bluestore_SharedBlob": {
"items": 17536894,
"bytes": 1964132128
},
"bluestore_inline_bl": {
"items": 967,
"bytes": 141154
},
"bluestore_fsck": {
"items": 0,
"bytes": 0
},
"bluestore_txc": {
"items": 20,
"bytes": 15680
},
"bluestore_writing_deferred": {
"items": 55,
"bytes": 427691
},
"bluestore_writing": {
"items": 45,
"bytes": 184320
},
"bluefs": {
"items": 16771,
"bytes": 249024
},
"bluefs_file_reader": {
"items": 352,
"bytes": 58242816
},
"bluefs_file_writer": {
"items": 3,
"bytes": 576
},
"buffer_anon": {
"items": 588578,
"bytes": 87927562
},
"buffer_meta": {
"items": 254277,
"bytes": 22376376
},
"osd": {
"items": 1367,
"bytes": 15463504
},
"osd_mapbl": {
"items": 0,
"bytes": 0
},
"osd_pglog": {
"items": 827739,
"bytes": 429467480
},
"osdmap": {
"items": 2794507,
"bytes": 43126616
},
"osdmap_mapping": {
"items": 0,
"bytes": 0
},
"pgmap": {
"items": 0,
"bytes": 0
},
"mds_co": {
"items": 0,
"bytes": 0
},
"unittest_1": {
"items": 0,
"bytes": 0
},
"unittest_2": {
"items": 0,
"bytes": 0
}
},
"total": {
"items": 205850092,
"bytes": 9282297783
}
}
}
The total bytes at the end is much less than what the OS reports. Is this something I can control by adjusting the calculation frequency as Mark suggests?
-- Sam Clippinger
-----Original Message-----
From: Mark Nelson <mnelson@xxxxxxxxxx>
Sent: Tuesday, March 29, 2022 1:27 PM
To: ceph-users@xxxxxxx
Subject: Re: What's the relationship between osd_memory_target and bluestore_cache_size?
CAUTION - EXTERNAL EMAIL: Do not click any links or open any attachments unless you trust the sender and know the content is safe.
On 3/29/22 11:44, Anthony D'Atri wrote:
>>> [osd]
>>> bluestore_cache_autotune = 0
>> Why are you turning autotuning off?
> FWIW I’ve encountered the below assertions. I neither support nor deny them, pasting here for discussion. One might interpret this to only apply to OSDs with DB on a seperate (faster) device.
>
>
>>>> With random small block workloads, it’s important to keep BlueStore metadata cached and keep RocksDB from spilling over to slow media – including during compaction. If there is adequate memory on the OSD node, it is recommended to increase the BlueStore metadata cache ratio. An example of this is shown below:
>>>>
>>>> bluestore_cache_meta_ratio = 0.8
>>>> bluestore_cache_kv_ratio = 0.2
>>>> osd bluestore_cache_size_ssd 6GB
>>>>
>>>> In Ceph Nautilus and above, the cache ratios are automatically tuned so it is recommended to first observe the relevant cache hit counters in BlueStore before manually setting these parameters. There is some disagreement regarding how effective the auto tuning is.
>>>>
>>>>> https://urldefense.com/v3/__https://ceph.io/community/bluestore-de
>>>>> fault-vs-tuned-performance-comparison/__;!!EJc4YC3iFmQ!CYgPlRbNINH
>>>>> H3PZk1a41xJtauKQiIwpBKjNR9Fl68exXEekavrBL0KbFuDgULyoyAV8$
>>>>> suggests that we still set
>>>>>
>>>>> bluestore_cache_size_ssd = 8GB with 12GB memory target.
Sorry, this is going to be a long... :)
The basic gist of it is that if you disable autotuning, the OSD will use a set "cache size" for various caches and then divvy the memory up between them based on the defined ratios. For bluestore that means the rocksdb block cache(s), bluestore onode cache, and bluestore buffer cache. IE in the above example that's 6GB with 80% going to onode "meta" cache, 20% going to the rocksdb block "kv" cache, and an implicit 0% being dedicated to bluestore buffer cache. This kind of setup tends to work best when you have a well defined workload and you know exactly how you want to tune the different cache sizes for optimal performance (often times giving a lot of the memory to onode cache for RBD for example). The amount of memory the OSD uses can float up and down and it tends to be a little easier on tcmalloc because because you aren't growing/shrinking the caches constantly trying to stay within a certain memory target.
When cache autotuning is enabled, the cache size is allowed to fluctuate based on the osd_memory_target and how much memory is mapped by the ceph-osd process as reported by tcmalloc. This is almost like using RSS memory as the target but not quite. The difference is that there is no guarantee that the kernel will reclaim freed memory soon (or at all), so RSS memory usage ends up being a really poor metric for trying to dynamically adjust memory targets (I tried with fairly comical results). This process of adjusting the caches based on a process level memory target seems to be harder on tcmalloc, probably because we're freeing a bunch of fragmented memory (say from the onode cache) while it's simultaneously trying to hand sequential chunks of memory out to something else (whatever is requesting memory and forcing us to go over target). We tend to oscillate around the memory target, though over all the system works fairly well if you are willing to accept up to ~20% memory spikes under heavy (write) workloads. You can tweak the behavior to more aggressively try to control this by increasing the frequency that we recalculate the memory target, but it's more CPU intensive and may overcompensate by releasing too much fragmented memory too quickly.
Enabling autotuning also enables the priority cache manager. Each cache subsystem will request memory at different priority targets (say pri0, pri1, etc). When autotuning is enabled the ratios no longer govern a global percentage of the cache, but instead govern a "fairshare" target at each priorirty level. Each cache is assigned at least it's ratio of the available memory at a given level. If a cache is assigned all of the memory it requests at that level, the prioirty cache manager will use left over memory to fulfill requests at that level by caches that want more memory than their faireshare target. This process continues until all requests at a given level have been fulfilled or we run out of memory available for caches. If all requests have been fulfilled at a given level, we move to the next level and start the process all over again.
In current versions of ceph we only really utilize 2 of the available levels. Priority0 is used for very high priority things (like items pinned in the cache or rocksdb "hipri pool" items. Everything else is basically shoved into a single level and competes there. In Quincy, we finally implemented age-binning, where we associate items in the different caches with "age bins" that give us a coarse look at the relative ages of all cache items. IE say that there are old onode entries sitting in the bluestore onode cache, but now there is a really hot read workload against a single large object. That OSD's priority cache can now sort those older onode entries into a lower priority level than the buffer cache data for the hot object. We generally may heavily favor onodes at a given priority level, but in this case older onodes may end up in a lower priority level than the hot buffer data, so the buffer data memory request is fulfilled first.
Due to various factors this isn't as big of a win as I had hoped it would be (primarily in relation to the rocksdb block cache, since compaction tends to blow everything in the cache away regularly anyway). In reality the biggest benefit seems to be that we are more aggressive about clearing away very old onode data if there are new writes which we suspect is reducing memory fragmentation, and it's much easier to tell the ages of items in the various caches via the perf admin socket. It does give us significantly more control and insight into the global cache behavior though, so in general it seems to be a good thing. The perf tests we ran ranged from having little effect to showing moderate improvement in some scenarios.
FWIW, despite the fact that I wrote the prioritycache system and memory autotuning code, I'd be much happier if we were much less dynamic about how we allocate memory. That probably goes all the way back to how the message over the wire looks. Ideally we would have a very short path from the message to the disk with minimal intermediate translation of the message, minimal dynamic behavior based on the content of the message, and recycling static buffers or objects from a contiguous pool whenever possible. The prioritycache system tries to account for dynamic memory allocations in ceph by reactively growing/shrinking the caches, but it would be much better if we didn't need to do any of that in the first place.
Mark
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