On Thu, 2008-01-17 at 18:13 +0900, FUJITA Tomonori wrote: > On Wed, 16 Jan 2008 14:35:50 +0200 > Benny Halevy <bhalevy@xxxxxxxxxxx> wrote: > > > On Jan. 15, 2008, 17:20 +0200, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, 2008-01-15 at 18:23 +0900, FUJITA Tomonori wrote: > > >> This is the second version of > > >> > > >> http://marc.info/?l=linux-scsi&m=119933628210006&w=2 > > >> > > >> I gave up once, but I found that the performance loss is negligible > > >> (within 1%) by using kmem_cache_alloc instead of mempool. > > >> > > >> I use scsi_debug with fake_rw=1 and disktest (DIO reads with 8 > > >> threads) again: > > >> > > >> scsi-misc (slub) | 486.9 MB/s IOPS 124652.9/s > > >> dynamic sense buf (slub) | 483.2 MB/s IOPS 123704.1/s > > >> > > >> scsi-misc (slab) | 467.0 MB/s IOPS 119544.3/s > > >> dynamic sense buf (slab) | 468.7 MB/s IOPS 119986.0/s > > >> > > >> The results are the averages of three runs with a server using two > > >> dual-core 1.60 GHz Xeon processors with DDR2 memory. > > >> > > >> > > >> I doubt think that someone will complain about the performance > > >> regression due to this patch. In addition, unlike scsi_debug, the real > > >> LLDs allocate the own data structure per scsi_cmnd so the performance > > >> differences would be smaller (and with the real hard disk overheads). > > >> > > >> Here's the full results: > > >> > > >> http://www.kernel.org/pub/linux/kernel/people/tomo/sense/results.txt > > > > > > Heh, that's one of those good news, bad news things. Certainly good > > > news for you. The bad news for the rest of us is that you just > > > implicated mempool in a performance problem and since they're the core > > > of the SCSI scatterlist allocations and sit at the heart of the critical > > > path in SCSI, we have a potential performance issue in the whole of > > > SCSI. > > > > Looking at mempool's code this is peculiar as what seems to be its > > critical path for alloc and free looks pretty harmless and lightweight. > > Maybe an extra memory barrier, spin_{,un}lock_* and two extra function call > > (one of them can be eliminated BTW if the order of arguments to the > > mempool_{alloc,free}_t functions were the same as for kmem_cache_{alloc,free}). > > Yeah, so I wondered why the change made a big difference. After more > testing, it turned out that mempool is not so slow. > > v1 patch reserves as many buffers as can_queue per shost. My test > server allocates 1519 sense buffers in total and then needs to > allocate more. Seems that it hurts the performance. I would bet it does. Mempools aren't a performance enhancer, they're a deadlock avoider. So you don't prefill them with 1519 entries per host, you prefill them with at most two so that we can always guarantee getting a writeout command down in the event the system is totally out of GFP_ATOMIC memory and needs to free something. Plus, pool allocations of that size will get me hunted down and shot by the linux tiny (or other embedded) community. > I modified v3 patch to allocate unused 1519 sense buffers via > kmem_cache_alloc. It achieved 96.2% of the scsi-misc performance (note > that v1 patch achieved 94.6% of the scsi-misc). > > I modified v3 patch to use mempool to allocate one buffer per host. It > achieved 98.3% of the scsi-misc (note that v3 patch achieved 99.3% of > the scsi-misc). This is about the correct thing to do. > So I could say: > > - mempool is only about 1% slower to using kmem_cache directly. > > - reserving lots of slabs seems to hurt SLUB performance (I've not dug > into it yet). > > > The full results and two patches are: > > http://www.kernel.org/pub/linux/kernel/people/tomo/sense/ James - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
