On Tue, Oct 17, 2017 at 10:49 AM, Sage Weil <sweil@xxxxxxxxxx> wrote: > On Tue, 17 Oct 2017, xiaoyan li wrote: >> Hi Sage and Mark, >> A question here: OMAP pg logs are added by "set", are they only >> deleted by rm_range_keys in BlueStore? >> https://github.com/ceph/ceph/pull/18279/files > > Ooh, I didn't realize we weren't doing this already--we should definitely > merge this patch. But: > >> If yes, maybe when dedup, we don't need to compare the keys in all >> memtables, we just compare keys in current memtable with rm_range_keys >> in later memtables? > > They are currently deleted explicitly by key name by the OSD code; it > doesn't call the range-based delete method. Radoslaw had a test branch > last week that tried using rm_range_keys instead but he didn't see any > real difference... presumably because we didn't realize the bluestore omap > code wasn't passing a range delete down to KeyValuDB! We should retest on > top of your change. I will also have a check. A memtable table includes two parts: key/value operations(set, delete, deletesingle, merge), and range_del(includes range delete). I am wondering if all the pg logs are deleted by range delete, we can just check whether a key/value is deleted in range_del parts of later memtables when dedup flush, this can be save a lot of comparison effort. > > Thanks! > sage > > > > > >> >> On Tue, Oct 17, 2017 at 10:18 AM, xiaoyan li <wisher2003@xxxxxxxxx> wrote: >> > Hi Sage and Mark, >> > Following tests results I give are tested based on KV sequences got >> > from librbd+fio 4k or 16k random writes in 30 mins. >> > In my opinion, we may use dedup flush style for onodes and deferred >> > data, but use default merge flush style for other data. >> > >> > On Mon, Oct 16, 2017 at 9:50 PM, Mark Nelson <mnelson@xxxxxxxxxx> wrote: >> >> >> >> >> >> On 10/16/2017 08:28 AM, Sage Weil wrote: >> >>> >> >>> [adding ceph-devel] >> >>> >> >>> On Mon, 16 Oct 2017, Mark Nelson wrote: >> >>>> >> >>>> Hi Lisa, >> >>>> >> >>>> Excellent testing! This is exactly what we were trying to understand. >> >>>> >> >>>> On 10/16/2017 12:55 AM, Li, Xiaoyan wrote: >> >>>>> >> >>>>> Hi Mark, >> >>>>> >> >>>>> Based on my testing, when setting min_write_buffer_number_to_merge as 2, >> >>>>> the >> >>>>> onodes and deferred data written into L0 SST can decreased a lot with my >> >>>>> rocksdb dedup package. >> >>>>> >> >>>>> But for omap data, it needs to span more memtables. I tested omap data >> >>>>> in >> >>>>> separate column family. From the data, you can see when >> >>>>> min_write_buffer_number_to_merge is set to 4, the data written into L0 >> >>>>> SST >> >>>>> is good. That means it has to compare current memTable to flush with >> >>>>> later 3 >> >>>>> memtables recursively. >> >>>>> kFlushStyleDedup is to new flush style in my rocksdb dedup package. >> >>>>> kFlushStyleMerge is current flush style in master branch. >> >>>>> >> >>>>> But this is just considered from data written into L0. With more >> >>>>> memtables >> >>>>> to compare, it sacrifices CPU and computing time. >> >>>>> >> >>>>> Memtable size: 256MB >> >>>>> max_write_buffer_number min_write_buffer_number_to_merge >> >>>>> flush_style Omap data written into L0 SST(unit: MB) >> >>>>> 16 8 kFlushStyleMerge 7665 >> >>>>> 16 8 kFlushStyleDedup 3770 >> >>>>> 8 4 kFlushStyleMerge 11470 >> >>>>> 8 4 kFlushStyleDedup 3922 >> >>>>> 6 3 kFlushStyleMerge 14059 >> >>>>> 6 3 kFlushStyleDedup 5001 >> >>>>> 4 2 kFlushStyleMerge 18683 >> >>>>> 4 2 kFlushStyleDedup 15394 >> >>>> >> >>>> >> >>>> Is this only omap data or all data? It looks like the 6/3 or 8/4 is >> >>>> still >> >>>> probably the optimal point (And the improvements are quite noticeable!). >> > This is only omap data. Dedup can decrease data written into L0 SST, >> > but it needs to compare too many memtables. >> > >> >>>> Sadly we were hoping we might be able to get away with smaller memtables >> >>>> (say >> >>>> 64MB) with KFlushStyleDedup. It looks like that might not be the case >> >>>> unless >> >>>> we increase the number very high. >> >>>> >> >>>> Sage, this is going to be even worse if we try to keep more pglog entries >> >>>> around on flash OSD backends? >> >>> >> >>> >> >>> I think there are three or more factors at play here: >> >>> >> >>> 1- If we reduce the memtable size, the CPU cost of insertion (baseline) >> >>> and the dedup cost will go down. >> >>> >> >>> 2- If we switch to a small min pg log entries, then most pg log keys >> >>> *will* fall into the smaller window (of small memtables * small >> >>> min_write_buffer_to_merge). The dup op keys probably won't, though... >> >>> except maybe they will because the values are small and more of them will >> >>> fit into the memtables. But then >> >>> >> >>> 3- If we have more keys and smaller values, then the CPU overhead will be >> >>> higher again. >> >>> >> >>> For PG logs, I didn't really expect that the dedup style would help; I was >> >>> only thinking about the deferred keys. I wonder if it would make sense to >> >>> specify a handful of key prefixes to attempt dedup on, and not bother on >> >>> the others? >> >> >> >> >> >> Deferred keys seem to be a much smaller part of the problem right now than >> >> pglog. At least based on what I'm seeing at the moment with NVMe testing. >> >> Regarding dedup, it's probably worth testing at the very least. >> > I did following tests: all data in default column family. Set >> > min_write_buffer_to_merge to 2, check the size of kinds of data >> > written into L0 SST files. >> > From the data, onodes and deferred data can be removed a lot in dedup style. >> > >> > Data written into L0 SST files: >> > >> > 4k random writes (unit: MB) >> > FlushStyle Omap onodes deferred others >> > merge 22431.56 23224.54 1530.105 0.906106 >> > dedup 22188.28 14161.18 12.68681 0.90906 >> > >> > 16k random writes (unit: MB) >> > FlushStyle Omap onodes deferred others >> > merge 19260.20 8230.02 0 1914.50 >> > dedup 19154.92 2603.90 0 >> > 2517.15 >> > >> > Note here: for others type, which use "merge" operation, dedup style >> > can't make it more efficient. In later, we can set it in separate CF, >> > use default merge flush style. >> > >> >> >> >>> >> >>> Also, there is the question of where the CPU time is spent. >> >> >> >> >> >> Indeed, but if we can reduce the memtable size it means we save CPU in other >> >> areas. Like you say below, it's complicated. >> >>> >> >>> >> >>> 1- Big memtables means we spend more time in submit_transaction, called by >> >>> the kv_sync_thread, which is a bottleneck. >> >> >> >> >> >> At least on NVMe we see it pretty regularly in the wallclock traces. I need >> >> to retest with Radoslav and Adam's hugepages PR to get a feel for how bad it >> >> is after that. >> >> >> >>> >> >>> 2- Higher dedup style flush CPU usage is spent in the compaction thread(s) >> >>> (I think?), which are asynchronous. >> >> >> >> >> >> L0 compaction is single threaded though so we must be careful.... >> >> >> >>> >> >>> At the end of the day I think we need to use less CPU total, so the >> >>> optimization of the above factors is a bit complicated. OTOH if the goal >> >>> is IOPS at whatever cost it'll probably mean a slightly different choice. >> >> >> >> >> >> I guess we should consider the trends. Lots of cores, lots of flash cells. >> >> How do we balance high throughput and low latency? >> >> >> >>> >> >>> I would *expect* that if we go from, say, 256mb tables to 64mb tables and >> >>> dedup of <= 4 of them, then we'll see a modest net reduction of total CPU >> >>> *and* a shift to the compaction threads. >> >> >> >> >> >> It seems like based on Lisa's test results that's too short lived? Maybe I'm >> >> not understanding what you mean? >> >> >> >>> >> >>> And changing the pg log min entries will counterintuitively increase the >> >>> costs of insertion and dedup flush because more keys will fit in the same >> >>> amount of memtable... but if we reduce the memtable size at the same time >> >>> we might get a win there too? Maybe? >> >> >> >> >> >> There's too much variability here to theorycraft it and your "maybe" >> >> statement confirms for me. ;) We need to get a better handle on what's >> >> going on. >> >> >> >>> >> >>> Lisa, do you think limiting the dedup check during flush to specific >> >>> prefixes would make sense as a general capability? If so, we could target >> >>> this *just* at the high-value keys (e.g., deferred writes) and avoid >> >>> incurring very much additional overhead for the key ranges that aren't >> >>> sure bets. >> > The easiest way to do it is to set data in different CFs, and use >> > different flush style(dedup or merge) in different CFs. >> > >> >> >> >> >> >> At least in my testing deferred writes during rbd 4k random writes are >> >> almost negligible: >> >> >> >> http://pad.ceph.com/p/performance_weekly >> >> >> >> I suspect it's all going to be about OMAP. We need a really big WAL that >> >> can keep OMAP around for a long time while quickly flushing object data into >> >> small memtables. On disk it's a big deal that this gets layed out >> >> sequentially but on flash I'm wondering if we'd be better off with a >> >> separate WAL for OMAP (a different rocksdb shard or different data store >> >> entirely). >> > Yes, OMAP data is main data written into L0 SST. >> > >> > Data written into every memtable: (uint: MB) >> > IO load omap ondes deferred others >> > 4k RW 37584 85253 323887 250 >> > 16k RW 33687 73458 0 3500 >> > >> > In merge flush style with min_buffer_to_merge=2. >> > Data written into every L0 SST: (unit MB) >> > IO load Omap onodes deferred others >> > 4k RW 22188.28 14161.18 12.68681 0.90906 >> > 16k RW 19260.20 8230.02 0 1914.50 >> > >> >> >> >> Mark >> >> >> >> >> >>> >> >>> sage >> >>> >> >>> >> >>>>> The above KV operation sequences come from 4k random writes in 30mins. >> >>>>> Overall, the Rocksdb dedup package can decrease the data written into L0 >> >>>>> SST, but it needs more comparison. In my opinion, whether to use dedup, >> >>>>> it >> >>>>> depends on the configuration of the OSD host: whether disk is over busy >> >>>>> or >> >>>>> CPU is over busy. >> >>>> >> >>>> >> >>>> Do you have any insight into how much CPU overhead it adds? >> >>>> >> >>>>> >> >>>>> Best wishes >> >>>>> Lisa >> >> >> >> -- >> >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >> >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> > >> > >> > >> > -- >> > Best wishes >> > Lisa >> >> >> >> -- >> Best wishes >> Lisa >> >> -- Best wishes Lisa -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
