2012/12/5, Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>: > Hi Namjae, > > How about set bdi->dirty_background_bytes according to bdi_thresh? I found > an issue during background flush process when review codes, if over > background > flush threshold, wb_check_background_flush will kick a work to current > per-bdi > flusher, but maybe it is other heavy dirties written in other bdis who > heavily > dirty pages instead of current bdi, the worst case is current bdi has many > frequently used data and flush lead to cache thresh. How about add a check > in wb_check_background_flush if it is not current bdi who contributes large > > number of dirty pages to background flush threshold(over > bdi->dirty_background_bytes), > then don't bother it. Hi Wanpeng. First, Thanks for your suggestion! Yes, I think that it looks reasonable. I will start checking it. Thanks. > > Regards, > Wanpeng Li > > On Tue, Nov 20, 2012 at 08:18:59AM +0900, Namjae Jeon wrote: >>2012/10/22, Dave Chinner <david@xxxxxxxxxxxxx>: >>> On Fri, Oct 19, 2012 at 04:51:05PM +0900, Namjae Jeon wrote: >>>> Hi Dave. >>>> >>>> Test Procedure: >>>> >>>> 1) Local USB disk WRITE speed on NFS server is ~25 MB/s >>>> >>>> 2) Run WRITE test(create 1 GB file) on NFS Client with default >>>> writeback settings on NFS Server. By default >>>> bdi->dirty_background_bytes = 0, that means no change in default >>>> writeback behaviour >>>> >>>> 3) Next we change bdi->dirty_background_bytes = 25 MB (almost equal to >>>> local USB disk write speed on NFS Server) >>>> *** only on NFS Server - not on NFS Client *** >>> >>> Ok, so the results look good, but it's not really addressing what I >>> was asking, though. A typical desktop PC has a disk that can do >>> 100MB/s and GbE, so I was expecting a test that showed throughput >>> close to GbE maximums at least (ie. around that 100MB/s). I have 3 >>> year old, low end, low power hardware (atom) that hanles twice the >>> throughput you are testing here, and most current consumer NAS >>> devices are more powerful than this. IOWs, I think the rates you are >>> testing at are probably too low even for the consumer NAS market to >>> consider relevant... >>> >>>> ---------------------------------------------------------------------------------- >>>> Multiple NFS Client test: >>>> ----------------------------------------------------------------------------------- >>>> Sorry - We could not arrange multiple PCs to verify this. >>>> So, we tried 1 NFS Server + 2 NFS Clients using 3 target boards: >>>> ARM Target + 512 MB RAM + ethernet - 100 Mbits/s, create 1 GB File >>> >>> But this really doesn't tells us anything - it's still only 100Mb/s, >>> which we'd expect is already getting very close to line rate even >>> with low powered client hardware. >>> >>> What I'm concerned about the NFS server "sweet spot" - a $10k server >>> that exports 20TB of storage and can sustain close to a GB/s of NFS >>> traffic over a single 10GbE link with tens to hundreds of clients. >>> 100MB/s and 10 clients is about the minimum needed to be able to >>> extrapolate a litle and make an informed guess of how it will scale >>> up.... >>> >>>> > 1. what's the comparison in performance to typical NFS >>>> > server writeback parameter tuning? i.e. dirty_background_ratio=5, >>>> > dirty_ratio=10, dirty_expire_centiseconds=1000, >>>> > dirty_writeback_centisecs=1? i.e. does this give change give any >>>> > benefit over the current common practice for configuring NFS >>>> > servers? >>>> >>>> Agreed, that above improvement in write speed can be achieved by >>>> tuning above write-back parameters. >>>> But if we change these settings, it will change write-back behavior >>>> system wide. >>>> On the other hand, if we change proposed per bdi setting, >>>> bdi->dirty_background_bytes it will change write-back behavior for the >>>> block device exported on NFS server. >>> >>> I already know what the difference between global vs per-bdi tuning >>> means. What I want to know is how your results compare >>> *numerically* to just having a tweaked global setting on a vanilla >>> kernel. i.e. is there really any performance benefit to per-bdi >>> configuration that cannot be gained by existing methods? >>> >>>> > 2. what happens when you have 10 clients all writing to the server >>>> > at once? Or a 100? NFS servers rarely have a single writer to a >>>> > single file at a time, so what impact does this change have on >>>> > multiple concurrent file write performance from multiple clients >>>> >>>> Sorry, we could not arrange more than 2 PCs for verifying this. >>> >>> Really? Well, perhaps there's some tools that might be useful for >>> you here: >>> >>> http://oss.sgi.com/projects/nfs/testtools/ >>> >>> "Weber >>> >>> Test load generator for NFS. Uses multiple threads, multiple >>> sockets and multiple IP addresses to simulate loads from many >>> machines, thus enabling testing of NFS server setups with larger >>> client counts than can be tested with physical infrastructure (or >>> Virtual Machine clients). Has been useful in automated NFS testing >>> and as a pinpoint NFS load generator tool for performance >>> development." >>> >> >>Hi Dave, >>We ran "weber" test on below setup: >>1) SATA HDD - Local WRITE speed ~120 MB/s, NFS WRITE speed ~90 MB/s >>2) Used 10GbE - network interface to mount NFS >> >>We ran "weber" test with NFS clients ranging from 1 to 100, >>below is the % GAIN in NFS WRITE speed with >>bdi->dirty_background_bytes = 100 MB at NFS server >> >>------------------------------------------------- >>| Number of NFS Clients |% GAIN in WRITE Speed | >>|-----------------------------------------------| >>| 1 | 19.83 % | >>|-----------------------------------------------| >>| 2 | 2.97 % | >>|-----------------------------------------------| >>| 3 | 2.01 % | >>|-----------------------------------------------| >>| 10 | 0.25 % | >>|-----------------------------------------------| >>| 20 | 0.23 % | >>|-----------------------------------------------| >>| 30 | 0.13 % | >>|-----------------------------------------------| >>| 100 | - 0.60 % | >>------------------------------------------------- >> >>with bdi->dirty_background_bytes setting at NFS server, we observed >>that NFS WRITE speed improvement is maximum with single NFS client. >>But WRITE speed improvement drops when Number of NFS clients increase >>from 1 to 100. >> >>So, bdi->dirty_background_bytes setting might be useful where we have >>only one NFS client(scenario like ours). >>But this is not useful for big NFS Servers which host hundreads of NFS >> clients. >> >>Let me know your opinion. >> >>Thanks. >> >>>> > 3. Following on from the multiple client test, what difference does >>>> > it >>>> > make to file fragmentation rates? Writing more frequently means >>>> > smaller allocations and writes, and that tends to lead to higher >>>> > fragmentation rates, especially when multiple files are being >>>> > written concurrently. Higher fragmentation also means lower >>>> > performance over time as fragmentation accelerates filesystem aging >>>> > effects on performance. IOWs, it may be faster when new, but it >>>> > will be slower 3 months down the track and that's a bad tradeoff to >>>> > make. >>>> >>>> We agree that there could be bit more framentation. But as you know, >>>> we are not changing writeback settings at NFS clients. >>>> So, write-back behavior on NFS client will not change - IO requests >>>> will be buffered at NFS client as per existing write-back behavior. >>> >>> I think you misunderstand - writeback settings on the server greatly >>> impact the way the server writes data and therefore the way files >>> are fragmented. It has nothing to do with client side tuning. >>> >>> Effectively, what you are presenting is best case numbers - empty >>> filesystem, single client, streaming write, no fragmentation, no >>> allocation contention, no competing IO load that causes write >>> latency occurring. Testing with lots of clients introduces all of >>> these things, and that will greatly impact server behaviour. >>> Aggregation in memory isolates a lot of this variation from >>> writeback and hence smooths out a lot of the variability that leads >>> to fragmentation, seeks, latency spikes and preamture filesystem >>> aging. >>> >>> That is, if you set a 100MB dirty_bytes limit on a bdi it will give >>> really good buffering for a single client doing a streaming write. >>> If you've got 10 clients, then assuming fair distribution of server >>> resources, then that is 10MB per client per writeback trigger. >>> That's line ball as to whether it will cause fragmentation severe >>> enough to impact server throughput. If you've got 100 clients,then >>> that's only 1MB per client per writeback trigger, and that's >>> definitely too low to maintain decent writeback behaviour. i.e. >>> you're now writing 100 files 1MB at a time, and that tends towards >>> random IO patterns rather than sequential IO patterns. Seek time >>> dertermines throughput, not IO bandwidth limits. >>> >>> IOWs, as the client count goes up, the writeback patterns will tends >>> more towards random IO than sequential IO unless the amount of >>> buffering allowed before writeback triggers also grows. That's >>> important, because random IO is much slower than sequential IO. >>> What I'd like to have is some insight into whether this patch >>> changes that inflection point, for better or for worse. The only way >>> to find that is to run multi-client testing.... >>> >>>> > 5. Are the improvements consistent across different filesystem >>>> > types? We've had writeback changes in the past cause improvements >>>> > on one filesystem but significant regressions on others. I'd >>>> > suggest that you need to present results for ext4, XFS and btrfs so >>>> > that we have a decent idea of what we can expect from the change to >>>> > the generic code. >>>> >>>> As mentioned in the above Table 1 & 2, performance gain in WRITE speed >>>> is different on different file systems i.e. different on NFS client >>>> over XFS & EXT4. >>>> We also tried BTRFS over NFS, but we could not see any WRITE speed >>>> performance gain/degrade on BTRFS over NFS, so we are not posting >>>> BTRFS results here. >>> >>> You should post btrfs numbers even if they show no change. It wasn't >>> until I got this far that I even realised that you'd even tested >>> BTRFS. I don't know what to make of this, because I don't know what >>> the throughput rates compared to XFS and EXT4 are.... >>> >>> Cheers, >>> >>> Dave. >>> -- >>> Dave Chinner >>> david@xxxxxxxxxxxxx >>> >>-- >>To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" >> 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 linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
