On Wed, Apr 9, 2014 at 6:51 PM, Zheng Liu <gnehzuil.liu@xxxxxxxxx> wrote: > On Wed, Apr 09, 2014 at 05:14:37PM +0800, liang xie wrote: >> Hi, >> >> I am an Apache HDFS/HBase developer and debugging the slow buffered io >> issue on ext4. I saw some slow sys_write caused by: >> (mount -o noatime) >> 0xffffffff814ed1c3 : io_schedule+0x73/0xc0 [kernel] >> 0xffffffff81110b4d : sync_page+0x3d/0x50 [kernel] >> 0xffffffff814eda2a : __wait_on_bit_lock+0x5a/0xc0 [kernel] >> 0xffffffff81110ae7 : __lock_page+0x67/0x70 [kernel] >> 0xffffffff81111abc : find_lock_page+0x4c/0x80 [kernel] >> 0xffffffff81111b3a : grab_cache_page_write_begin+0x4a/0xc0 [kernel] >> 0xffffffffa00d05d4 : ext4_da_write_begin+0xb4/0x200 [ext4] > > Delalloc obviously could cause a latency spike because of i_data_sem. > When flusher thread tries to write out some dirty pages, it will grab > i_data_sem locking and allocate some blocks for these dirty pages. At > that time if an application tries to do some buffered writes, i_data_sem > also need to be taken. So the application needs to wait on writeback. > Cool, got it :) >> >> seems caused by delay allocation, right? so i reran with "mount -o >> noatime,,nodiratime,data=writeback,nodelalloc", unfortunately, i saw >> another stack trace contributing high latency: >> 0xffffffff811a9416 : __wait_on_buffer+0x26/0x30 [kernel] >> 0xffffffffa0123564 : ext4_mb_init_cache+0x234/0x9f0 [ext4] >> 0xffffffffa0123e3e : ext4_mb_init_group+0x11e/0x210 [ext4] >> 0xffffffffa0123ffd : ext4_mb_good_group+0xcd/0x110 [ext4] >> 0xffffffffa01276eb : ext4_mb_regular_allocator+0x19b/0x410 [ext4] >> 0xffffffffa0127ced : ext4_mb_new_blocks+0x38d/0x560 [ext4] >> 0xffffffffa011dfc3 : ext4_ext_get_blocks+0x1113/0x1a10 [ext4] >> 0xffffffffa00fb335 : ext4_get_blocks+0xf5/0x2a0 [ext4] >> 0xffffffffa00fbdad : ext4_get_block+0xbd/0x120 [ext4] >> 0xffffffff811ab27b : __block_prepare_write+0x1db/0x570 [kernel] >> 0xffffffff811ab8cc : block_write_begin_newtrunc+0x5c/0xd0 [kernel] >> 0xffffffff811abcd3 : block_write_begin+0x43/0x90 [kernel] >> 0xffffffffa00fe408 : ext4_write_begin+0x1b8/0x2d0 [ext4] >> and from HDFS/HBASE side, also no obvious improvement be found. > > From the output of calltrace, it seems that we wait on reading some meta > data for block allocation. Any ideas on relieving the write stall caused by it? > >> >> and inside both two scenarios, the following stack trace was hit as well: >> 0xffffffffa00dc09d : do_get_write_access+0x29d/0x520 [jbd2] >> 0xffffffffa00dc471 : jbd2_journal_get_write_access+0x31/0x50 [jbd2] >> 0xffffffffa011eb78 : __ext4_journal_get_write_access+0x38/0x80 [ext4] >> 0xffffffffa01209ba : ext4_mb_mark_diskspace_used+0x7a/0x300 [ext4] >> 0xffffffffa0127c09 : ext4_mb_new_blocks+0x2a9/0x560 [ext4] >> 0xffffffffa011dfc3 : ext4_ext_get_blocks+0x1113/0x1a10 [ext4] >> 0xffffffffa00fb335 : ext4_get_blocks+0xf5/0x2a0 [ext4] >> 0xffffffffa00fbdad : ext4_get_block+0xbd/0x120 [ext4] >> >> My question is: >> 1)what's the ext4 best practice for low latency append-only workload >> like HBase application? Is there any recommended option i could try, >> flex_bg size? nomballoc? > > We do the following things in our product system in order to avoid > latency spike: > 1. -o nodelalloc > 2. -o data=writeback > 3. disable stable page write ok >> 2)for the last strace trace, does >> 9f203507ed277ee86e3f76a15e09db1c92e40b94 help a lot, or no big win? (i >> haven't run on 3.10+ so far and it's inconvenient to bump kernel >> version on my cluster currently, so forgive my this stupid question if >> it's...) > > TBH, I don't know. But it is not very hard to backport this patch into > your kernel. > > BTW, as far as I understand, Hadoop just does some parallel append buffer > writes, right? not exactly, per my current understanding, the hdfs data files are always append only write, but the meta files are not the same story, it possible has a minor overwrite request under special conditions. > Could you please write a simple program to reproduce this problem? np, will do once get chance Thanks for your nice reply, zheng :) > > Regards, > - Zheng -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
