Chris, Firstly inform the linux-embedded maintainers :) I think it's a good suggestion to add a config option (CONFIG_READAHEAD_SIZE). Will update the patch.. Thanks, Fengguang On Mon, Feb 08, 2010 at 03:20:31PM +0800, Christian Ehrhardt wrote: > This is related to our discussion from October 09 e.g. > http://lkml.indiana.edu/hypermail/linux/kernel/0910.1/01468.html > > I work for s390 where - as mainframe - we only have environments that > benefit from 512k readahead, but I still expect some embedded devices won't. > While my idea of making it configurable was not liked in the past, it > may be still useful when introducing this default change to let some > small devices choose without patching the src (a number field defaulting > to 512 and explaining the past of that value would be really nice). > > For the discussion of 512 vs. 128 I can add from my measurements that I > have seen the following: > - 512 is by far superior to 128 for sequential reads > - improvements with iozone sequential read scaling from 1 to 64 parallel > processes up to +35% > - readahead sizes larger than 512 reevealed to not be "more useful" but > increasing the chance of trashing in low mem systems > > So I appreciate this change with a little note that I would prefer a > config option. > -> tested & acked-by Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx> > > Wu Fengguang wrote: > > > > Use 512kb max readahead size, and 32kb min readahead size. > > > > The former helps io performance for common workloads. > > The latter will be used in the thrashing safe context readahead. > > > > -- Rationals on the 512kb size -- > > > > I believe it yields more I/O throughput without noticeably increasing > > I/O latency for today's HDD. > > > > For example, for a 100MB/s and 8ms access time HDD, its random IO or > > highly concurrent sequential IO would in theory be: > > > > io_size KB access_time transfer_time io_latency util% > throughput KB/s > > 4 8 0.04 8.04 0.49% 497.57 > > 8 8 0.08 8.08 0.97% 990.33 > > 16 8 0.16 8.16 1.92% 1961.69 > > 32 8 0.31 8.31 3.76% 3849.62 > > 64 8 0.62 8.62 7.25% 7420.29 > > 128 8 1.25 9.25 13.51% 13837.84 > > 256 8 2.50 10.50 23.81% 24380.95 > > 512 8 5.00 13.00 38.46% 39384.62 > > 1024 8 10.00 18.00 55.56% 56888.89 > > 2048 8 20.00 28.00 71.43% 73142.86 > > 4096 8 40.00 48.00 83.33% 85333.33 > > > > The 128KB => 512KB readahead size boosts IO throughput from ~13MB/s to > > ~39MB/s, while merely increases (minimal) IO latency from 9.25ms to 13ms. > > > > As for SSD, I find that Intel X25-M SSD desires large readahead size > > even for sequential reads: > > > > rasize 1st run 2nd run > > ---------------------------------- > > 4k 123 MB/s 122 MB/s > > 16k 153 MB/s 153 MB/s > > 32k 161 MB/s 162 MB/s > > 64k 167 MB/s 168 MB/s > > 128k 197 MB/s 197 MB/s > > 256k 217 MB/s 217 MB/s > > 512k 238 MB/s 234 MB/s > > 1M 251 MB/s 248 MB/s > > 2M 259 MB/s 257 MB/s > > 4M 269 MB/s 264 MB/s > > 8M 266 MB/s 266 MB/s > > > > The two other impacts of an enlarged readahead size are > > > > - memory footprint (caused by readahead miss) > > Sequential readahead hit ratio is pretty high regardless of max > > readahead size; the extra memory footprint is mainly caused by > > enlarged mmap read-around. > > I measured my desktop: > > - under Xwindow: > > 128KB readahead hit ratio = 143MB/230MB = 62% > > 512KB readahead hit ratio = 138MB/248MB = 55% > > 1MB readahead hit ratio = 130MB/253MB = 51% > > - under console: (seems more stable than the Xwindow data) > > 128KB readahead hit ratio = 30MB/56MB = 53% > > 1MB readahead hit ratio = 30MB/59MB = 51% > > So the impact to memory footprint looks acceptable. > > > > - readahead thrashing > > It will now cost 1MB readahead buffer per stream. Memory tight > > systems typically do not run multiple streams; but if they do > > so, it should help I/O performance as long as we can avoid > > thrashing, which can be achieved with the following patches. > > > > -- Benchmarks by Vivek Goyal -- > > > > I have got two paths to the HP EVA and got multipath device setup(dm-3). > > I run increasing number of sequential readers. File system is ext3 and > > filesize is 1G. > > I have run the tests 3 times (3sets) and taken the average of it. > > > > Workload=bsr iosched=cfq Filesz=1G bs=32K > > ====================================================================== > > 2.6.33-rc5 2.6.33-rc5-readahead > > job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s) MaxClat(us) > > --- --- -- ------------ ----------- ------------ ----------- > > bsr 3 1 141768 130965 190302 97937.3 > > bsr 3 2 131979 135402 185636 223286 > > bsr 3 4 132351 420733 185986 363658 > > bsr 3 8 133152 455434 184352 428478 > > bsr 3 16 130316 674499 185646 594311 > > > > I ran same test on a different piece of hardware. There are few SATA > disks > > (5-6) in striped configuration behind a hardware RAID controller. > > > > Workload=bsr iosched=cfq Filesz=1G bs=32K > > ====================================================================== > > 2.6.33-rc5 2.6.33-rc5-readahead > > job Set NR ReadBW(KB/s) MaxClat(us) ReadBW(KB/s) > MaxClat(us) > > --- --- -- ------------ ----------- ------------ > ----------- > > bsr 3 1 147569 14369.7 160191 > 22752 > > bsr 3 2 124716 243932 149343 > 184698 > > bsr 3 4 123451 327665 147183 > 430875 > > bsr 3 8 122486 455102 144568 > 484045 > > bsr 3 16 117645 1.03957e+06 137485 > 1.06257e+06 > > > > Tested-by: Vivek Goyal <vgoyal@xxxxxxxxxx> > > CC: Jens Axboe <jens.axboe@xxxxxxxxxx> > > CC: Chris Mason <chris.mason@xxxxxxxxxx> > > CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > > CC: Martin Schwidefsky <schwidefsky@xxxxxxxxxx> > > CC: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx> > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > > --- > > include/linux/mm.h | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > --- linux.orig/include/linux/mm.h 2010-01-30 17:38:49.000000000 +0800 > > +++ linux/include/linux/mm.h 2010-01-30 18:09:58.000000000 +0800 > > @@ -1184,8 +1184,8 @@ int write_one_page(struct page *page, in > > void task_dirty_inc(struct task_struct *tsk); > > > > /* readahead.c */ > > -#define VM_MAX_READAHEAD 128 /* kbytes */ > > -#define VM_MIN_READAHEAD 16 /* kbytes (includes current page) */ > > +#define VM_MAX_READAHEAD 512 /* kbytes */ > > +#define VM_MIN_READAHEAD 32 /* kbytes (includes current page) */ > > > > int force_page_cache_readahead(struct address_space *mapping, struct > file *filp, > > pgoff_t offset, unsigned long nr_to_read); > > > > > > -- > > Grüsse / regards, Christian Ehrhardt > IBM Linux Technology Center, Open Virtualization -- 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
