Thank you, Radek. Sage, I am going to use the patch to do test, and check whether we can just compare rang_del in Rocksdb when dedup recursively. On Wed, Oct 18, 2017 at 5:59 PM, Radoslaw Zarzynski <rzarzyns@xxxxxxxxxx> wrote: > Hello Sage, > > the patch is at my Github [1]. > > Please be aware we also need to set "rocksdb_enable_rmrange = true" to > use the DeleteRange of rocksdb::WriteBatch's interface. Otherwise our KV > abstraction layer would translate rm_range_keys() into a sequence of calls > to Delete(). > > Regards, > Radek > > P.S. > My apologies for duplicating the message. > > [1] https://github.com/ceph/ceph/commit/92a28f033a5272b7dc2c5d726e67b6d09f6166ba > > On Tue, Oct 17, 2017 at 5:21 AM, Sage Weil <sweil@xxxxxxxxxx> wrote: >> On Tue, 17 Oct 2017, xiaoyan li wrote: >>> 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. >> >> That sounds very promising! Radoslaw, can you share your patch changing >> the PG log trimming behavior? >> >> Thanks! >> sage >> >>> >>> > >>> > 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 >>> >>> -- 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
