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}). Benny > > 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
