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 > > -- 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
