Do you have the SSTables per read metric for before and after you increased the key cache size? If it was high before, that may have been the culprit meaning compaction tuning is in order.
On Fri, Aug 25, 2023, 12:35 PM Shaurya Gupta <shaurya.n...@gmail.com> wrote: > Thanks everyone. > Updating this thread - > We increased the key cache size from 100 MB to 200 MB and we believe that > has brought down the latency from 40 ms p95 to 6 ms p95. I think there is > still scope for improvement as both writes and reads are presently at p95 6 > ms. I would expect writes to be lower. But we are good with 6 ms for now at > least. > > On Mon, Aug 14, 2023 at 11:56 AM Elliott Sims via user < > user@cassandra.apache.org> wrote: > >> 1. Check for Nagle/delayed-ack, but probably nodelay is getting set by >> the driver so it shouldn't be a problem. >> 2. Check for network latency (just regular old ping among hosts, during >> traffic) >> 3. Check your GC metrics and see if garbage collections line up with >> outliers. Some tuning can help there, depending on the pattern, but 40ms >> p99 at least would be fairly normal for G1GC. >> 4. Check actual local write times, and I/O times with iostat. If you >> have spinning drives 40ms is fairly expected. It's high but not totally >> unexpected for consumer-grade SSDs. For enterprise-grade SSDs commit times >> that long would be very unusual. What are your commitlog_sync settings? >> >> On Mon, Aug 14, 2023 at 8:43 AM Josh McKenzie <jmcken...@apache.org> >> wrote: >> >>> The queries are rightly designed >>> >>> Data modeling in Cassandra is 100% gray space; there unfortunately is no >>> right or wrong design. You'll need to share basic shapes / contours of your >>> data model for other folks to help you; seemingly innocuous things in a >>> data model can cause unexpected issues w/C*'s storage engine paradigm >>> thanks to the partitioning and data storage happening under the hood. >>> >>> If you were seeing single digit ms on 3.0.X or 3.11.X and 40ms p95 on >>> 4.0 I'd immediately look to the DB as being the culprit. For all other >>> cases, you should be seeing single digit ms as queries in C* generally boil >>> down to key/value lookups (partition key) to a list of rows you either >>> point query (key/value #2) or range scan via clustering keys and pull back >>> out. >>> >>> There's also paging to take into consideration (whether you're using it >>> or not, what your page size is) and the data itself (do you have thousands >>> of columns? Multi-MB blobs you're pulling back out? etc). All can play into >>> this. >>> >>> On Fri, Aug 11, 2023, at 3:40 PM, Jeff Jirsa wrote: >>> >>> You’re going to have to help us help you >>> >>> 4.0 is pretty widely deployed. I’m not aware of a perf regression >>> >>> Can you give us a schema (anonymized) and queries and show us a trace ? >>> >>> >>> On Aug 10, 2023, at 10:18 PM, Shaurya Gupta <shaurya.n...@gmail.com> >>> wrote: >>> >>> >>> The queries are rightly designed as I already explained. 40 ms is way >>> too high as compared to what I seen with other DBs and many a times with >>> Cassandra 3.x versions. >>> CPU consumed as I mentioned is not high, it is around 20%. >>> >>> On Thu, Aug 10, 2023 at 5:14 PM MyWorld <timeplus.1...@gmail.com> wrote: >>> >>> Hi, >>> P95 should not be a problem if rightly designed. Levelled compaction >>> strategy further reduces this, however it consume some resources. For read, >>> caching is also helpful. >>> Can you check your cpu iowait as it could be the reason for delay >>> >>> Regards, >>> Ashish >>> >>> On Fri, 11 Aug, 2023, 04:58 Shaurya Gupta, <shaurya.n...@gmail.com> >>> wrote: >>> >>> Hi community >>> >>> What is the expected P95 latency for Cassandra Read and Write queries >>> executed with Local_Quorum over a table with 3 replicas ? The queries are >>> done using the partition + clustering key and row size in bytes is not too >>> much, maybe 1-2 KB maximum. >>> Assuming CPU is not a crunch ? >>> >>> We observe those to be 40 ms P95 Reads and same for Writes. This looks >>> very high as compared to what we expected. We are using Cassandra 4.0. >>> >>> Any documentation / numbers will be helpful. >>> >>> Thanks >>> -- >>> Shaurya Gupta >>> >>> >>> >>> -- >>> Shaurya Gupta >>> >>> >>> >> This email, including its contents and any attachment(s), may contain >> confidential and/or proprietary information and is solely for the review >> and use of the intended recipient(s). If you have received this email in >> error, please notify the sender and permanently delete this email, its >> content, and any attachment(s). Any disclosure, copying, or taking of any >> action in reliance on an email received in error is strictly prohibited. >> > > > -- > Shaurya Gupta > > >
