path: root/block
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authorDivyesh Shah <>2008-06-16 18:37:08 +0200
committerJens Axboe <>2008-07-01 09:06:42 +0200
commitd585d0b9d73ed999cc7b8cf3cac4a5b01abb544e (patch)
tree7cfb65304f92745eb7ca3e5fa9073bbba1cfa357 /block
parent1702b52092e9a6d05398d3f9581ddc050ef00d06 (diff)
block: Fix the starving writes bug in the anticipatory IO scheduler
AS scheduler alternates between issuing read and write batches. It does the batch switch only after all requests from the previous batch are completed. When switching to a write batch, if there is an on-going read request, it waits for its completion and indicates its intention of switching by setting ad->changed_batch and the new direction but does not update the batch_expire_time for the new write batch which it does in the case of no previous pending requests. On completion of the read request, it sees that we were waiting for the switch and schedules work for kblockd right away and resets the ad->changed_data flag. Now when kblockd enters dispatch_request where it is expected to pick up a write request, it in turn ends the write batch because the batch_expire_timer was not updated and shows the expire timestamp for the previous batch. This results in the write starvation for all the cases where there is the intention for switching to a write batch, but there is a previous in-flight read request and the batch gets reverted to a read_batch right away. This also holds true in the reverse case (switching from a write batch to a read batch with an in-flight write request). I've checked that this bug exists on 2.6.11, 2.6.18, 2.6.24 and linux-2.6-block git HEAD. I've tested the fix on x86 platforms with SCSI drives where the driver asks for the next request while a current request is in-flight. This patch is based off linux-2.6-block git HEAD. Bug reproduction: A simple scenario which reproduces this bug is: - dd if=/dev/hda3 of=/dev/null & - lilo The lilo takes forever to complete. This can also be reproduced fairly easily with the earlier dd and another test program doing msync(). The example test program below should print out a message after every iteration but it simply hangs forever. With this bugfix it makes forward progress. ==== Example test program using msync() (thanks to suleiman AT google DOT com) inline uint64_t rdtsc(void) { int64_t tsc; __asm __volatile("rdtsc" : "=A" (tsc)); return (tsc); } int main(int argc, char **argv) { struct stat st; uint64_t e, s, t; char *p, q; long i; int fd; if (argc < 2) { printf("Usage: %s <file>\n", argv[0]); return (1); } if ((fd = open(argv[1], O_RDWR | O_NOATIME)) < 0) err(1, "open"); if (fstat(fd, &st) < 0) err(1, "fstat"); p = mmap(NULL, st.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); t = 0; for (i = 0; i < 1000; i++) { *p = 0; msync(p, 4096, MS_SYNC); s = rdtsc(); *p = 0; __asm __volatile(""::: "memory"); e = rdtsc(); if (argc > 2) printf("%d: %lld cycles %jd %jd\n", i, e - s, (intmax_t)s, (intmax_t)e); t += e - s; } printf("average time: %lld cycles\n", t / 1000); return (0); } Cc: <> Acked-by: Nick Piggin <> Signed-off-by: Jens Axboe <>
Diffstat (limited to 'block')
1 files changed, 2 insertions, 0 deletions
diff --git a/block/as-iosched.c b/block/as-iosched.c
index 8c3946787db..743f33a01a0 100644
--- a/block/as-iosched.c
+++ b/block/as-iosched.c
@@ -831,6 +831,8 @@ static void as_completed_request(struct request_queue *q, struct request *rq)
if (ad->changed_batch && ad->nr_dispatched == 1) {
+ ad->current_batch_expires = jiffies +
+ ad->batch_expire[ad->batch_data_dir];
ad->changed_batch = 0;