Details
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Improvement
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Resolution: Fixed
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Major
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None
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8137
Description
Perform short I/O (requests <= 4k) w/o bulk RPC.
Attachments
Issue Links
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LU-12856 LustreError: 82937:0:(ldlm_lib.c:3268:target_bulk_io()) @@@ truncated bulk READ 0(270336)
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- Resolved
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- Resolved
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LU-10289
DoM: add SHORTIO support for MDS RPCs
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- Resolved
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- Resolved
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Activity
Patrick, have you tested aligned or unaligned reads/writes? I expect with unaligned multi-client writes and server-side locking that this could also improve performance significantly.
There could also be a big benefit from bypassing the client-side aggregation and caching mechanisms completely in that case, and just dump the chunks to the OST as fast as possible, and use something like NRS ORR to aggregate the IOs properly on the server, or at least avoid read-modify-write for small writes over the network.
I've resurrected this patch and ported to current master. Some simple testing here suggests A) it's working fine, and B) it gives about a 30% performance improvement for direct I/O of appropriate size (I upped the limit to 3 pages, that's what fits in the RPC, I believe), when I'm reading from a fast storage device (RAM or flash). When I'm doing small I/O to/from a spinning disk, I see no real improvement - But that's probably because network latency is not the primary driver of I/O performance there.
Patrick Farrell (paf@cray.com) uploaded a new patch: https://review.whamcloud.com/27767
Subject: LU-1757 brw: add short io osc/ost transfer.
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 01056e12846a73c041da92d8a4f216f2641ca1cc
I want to reserve OBDO flag for short io
http://review.whamcloud.com/8182
Andreas, right now, I have no time and resources to check your suggestion, and the short io patch is outdated for master and required reworks. Data-on-MDT looks very good for short io and need another patch also.
We did not do the lockless test for shared files, but single client pages write with oflags=direct did not show significant improvement.
Alexander, I saw that the patch for this feature is abandoned, however small writes is definitely an area that Lustre could use a considerable amount of improvement. I'm still hopeful that there may be some workloads that this feature could show significant performance improvements on, or at least show what other work needs to be done in addition to this patch. The Data-on-MDT work is more concerned with small files and is definitely orthogonal to this small write patch which is intended to improve small write RPCs to a potentially very large file.
It may be that we need to make additional changes in order to see the overall improvement of small files. Some areas to investigate to see why this patch isn't showing the expected improvements:
- what is the improvement when the writes are smaller than a single disk block?
- what is the improvement when multiple clients are doing interleaved writes to the same file? This can be tested relatively easily with IOR and multiple client nodes ("ior -w -b 32 -t 32 -s 65536 -N 8 -i 10 -o /mnt/lustre/testfile" runs on 8 clients and does 65536 interleaved 32-byte writes per client).
- what impact does NRS object-based round-robin (ORR) have when doing small writing to a single file? This should sort the writes by file offset, but it may be that short writes also need to be cached on the OST so that they can avoid synchronous read-modify-write on the disk. This might be more easily tested with a ZFS OSD, which already does write caching, while the ldiskfs OSD would need changes to the IO path in order to cache small writes.
- in the shared single interleaved write case, are the clients doing lockless writes? If not, the lock contention and overhead of doing LDLM enqueue/cancel for each write may easily dominate over the improvement from the small write patch. For sub-page writes, it might also be that there needs to be some IO fastpath that bypasses the client page cache so that it can avoid read-modify-write for the local page.
I suspect that if there were multiple clients doing small writes
I was thinking of another potential area where this short IO could improve performance significantly (and give a good reason to land it), is when many clients are writing to the same object. Is it possible for you to run a test with multiple clients IOR writing <= 4kB interleaved chunks to the same 1-stripe file? Ideally this would use server-side locking for the writes, so that there is very minimal contention. It might even be that submitting smaller IOs (say 32 bytes) would give even more of a boost to this patch, since the client does not need to do read-modify-write for the full-page writes as it does today.
If this feature can show some significant performance improvements (say 3-4 times faster, though I'd expect possibly much more) then I would be happy to work on getting this this feature landed.
> In talking with Eric, one concern with using inline bulk data is that this can increase the request size enough to cause the routers to use 1MB buffers for handling the short IO requests, and potentially cause the routers to run out of buffers.
That is too bad for routers. Routers should be have more then two sizes for request size, anyway we have send a transfer size as part of lnet header.
I've tried several times over the past couple weeks to test this patch with master over the WAN but every time I do direct IO read or write I get an LBUG:
[root@test tmp]# dd if=test of=/dev/null bs=4k iflag=direct
Message from syslogd@test at Nov 27 03:44:12 ...
kernel:LustreError: 19403:0:(rw26.c:483:ll_direct_IO_26()) ASSERTION( obj->cob_transient_pages == 0 ) failed:
Is this an already known issue with direct IO on master?
Alex, if you want to run a test 10 times and get the average, I recommend ior's "-i" option. Otherwise its less obvious to others that you did more than eye-ball the numbers and pick a psuedo-average. With a write performance difference of only 2%, and overall throughput numbers that are so low, it is hard to tell if the results are statistically significant.
Andreas,
Aligned, mostly. Can't really test unaligned without getting aggregation or readahead, except for random reads. (Those do well.)
Server side locking... How is that achieved, other than with the patch from
LU-4198, and that only for direct i/o? (https://review.whamcloud.com/#/c/8201/20)And since it's direct i/o, it has to be aligned. (Trying to update
LU-247(unaligned dio) is my next project.)Also, other than with direct i/o, I'm not sure how to actually achieve small i/os (write aggregation or read ahead will prevent them), except for random reads. (Which do see a benefit - I didn't mention that, but they see the same sort of improvement.)
So, I suppose I would say:
I want to try all of that, I agree that it would likely benefit enormously (you pointed this out in your earlier comments to this LU) but I believe I need
LU-247to make it really possible, since direct i/o is the only way I know of to A) skip the page cache, B) force small i/o, and C) move the locking to the server (I can fake the effect of that by doing my i/o from one node.)Is there some easier route I've missed?