> One assumption would be that Postgres is perfectly happy with the current > kernel behaviour in which case our discussion here is done.
It has been demonstrated that this statement was farcical. The thread is massive just from interaction with the LSF/MM program committee. I'm hoping that there will be Postgres representation at LSF/MM this year to bring the issues to a wider audience. I expect that LSF/MM can only commit to one person attending the whole summit due to limited seats but we could be more more flexible for the Postgres track itself so informal meetings can be arranged for the evenings and at collab summit. In this gets forgotten, this mail describes what has already been discussed and some of the proposals. Some stuff I do not describe because it was superseded by later discussion. If I missed something important, misinterpreted or simply screwed up then shout and I'll update this. I'd rather none of this gets lost even if it takes months or years to address it all. On testing of modern kernels ---------------------------- Josh Berkus claims that most people are using Postgres with 2.6.19 and consequently there may be poor awareness of recent kernel developments. This is a disturbingly large window of opportunity for problems to have been introduced. It begs the question what sort of penetration modern distributions shipping Postgres has. More information on why older kernels dominate in Postgres installation would be nice. Postgres bug reports and LKML ------------------------------- It is claimed that LKML does not welcome bug reports but it's less clear what the basis of this claim is. Is it because the reports are ignored? A possible explanation is that they are simply getting lost in the LKML noise and there would be better luck if the bug report was cc'd to a specific subsystem list. Another explanation is that there is not enough data available to debug the problem. The worst explanation is that to date the problem has not been fixable but the details of this have been lost and are now unknown. Is is possible that some of these bug reports can be refreshed so at least there is a chance they get addressed? Apparently there were changes to the reclaim algorithms that crippled performance without any sysctls. The problem may be compounded by the introduction of adaptive replacement cache in the shape of the thrash detection patches currently being reviewed. Postgres investigated the use of ARC in the past and ultimately abandoned it. Details are in the archives (http://www.Postgres.org/search/?m=1&q=arc&l=1&d=-1&s=r). I have not read then, just noting they exist for future reference. Sysctls to control VM behaviour are not popular as such tuning parameters are often used as an excuse to not properly fix the problem. Would it be possible to describe a test case that shows 2.6.19 performing well and a modern kernel failing? That would give the VM people a concrete basis to work from to either fix the problem or identify exactly what sysctls are required to make this work. I am confident that any bug related to VM reclaim in this area has been lost. At least, I recall no instances of it being discussed on linux-mm and it has not featured on LSF/MM during the last years. IO Scheduling ------------- Kevin Grittner has stated that it is known that the DEADLINE and NOOP schedulers perform better than any alternatives for most database loads. It would be desirable to quantify this for some test case and see can the default scheduler cope in some way. The deadline scheduler makes sense to a large extent though. Postgres is sensitive to large latencies due to IO write spikes. It is at least plausible that deadline would give more deterministic behaviour for parallel reads in the presence of large writes assuming there were not ordering problems between the reads/writes and the underlying filesystem. For reference, these IO spikes can be massive. If the shared buffer is completely dirtied in a short space of time then it could be 20-25% of RAM being dirtied and writeback required in typical configurations. There have been cases where it was worked around by limiting the size of the shared buffer to a small enough size so that it can be written back quickly. There are other tuning options available such as altering when dirty background writing starts within the kernel but that will not help if the dirtying happens in a very short space of time. Dave Chinner described the considerations as follows There's no absolute rule here, but the threshold for background writeback needs to consider the amount of dirty data being generated, the rate at which it can be retired and the checkpoint period the application is configured with. i.e. it needs to be slow enough to not cause serious read IO perturbations, but still fast enough that it avoids peaks at synchronisation points. And most importantly, it needs to be fast enought that it can complete writeback of all the dirty data in a checkpoint before the next checkpoint is triggered. In general, I find that threshold to be somewhere around 2-5s worth of data writeback - enough to keep a good amount of write combining and the IO pipeline full as work is done, but no more. e.g. if your workload results in writeback rates of 500MB/s, then I'd be setting the dirty limit somewhere around 1-2GB as an initial guess. It's basically a simple trade off buffering space for writeback latency. Some applications perform well with increased buffering space (e.g. 10-20s of writeback) while others perform better with extremely low writeback latency (e.g. 0.5-1s). Some of this may have been addressed in recent changes with IO-less dirty throttling. When considering stalls related to excessive IO it will be important to check if the kernel was later than 3.2 and what the underlying filesystem was. Again, it really should be possible to demonstrate this with a test case, one driven by pgbench maybe? Workload would generate a bunch of test data, dirty a large percentage of it and try to sync. Metrics would be measuring average read-only query latency when reading in parallel to the write, average latencies from the underlying storage, IO queue lengths etc and comparing default IO scheduler with deadline or noop. NUMA Optimisations ------------------ The primary one that showed up was zone_reclaim_mode. Enabling that parameter is a disaster for many workloads and apparently Postgres is one. It might be time to revisit leaving that thing disabled by default and explicitly requiring that NUMA-aware workloads that are correctly partitioned enable it. Otherwise NUMA considerations are not that much of a concern right now. Direct IO, buffered IO and double buffering ------------------------------------------- The general position of Postgres is that the kernel knows more about storage geometries and IO scheduling that an application can or should know. It would be preferred to have interfaces that allow Postgres. give hints to the kernel about how and when data should be written back. The alternative is exposing details of the underlying storage to userspace so Postgres can implement a full IO scheduler using direct IO. It has been asserted on the kernel side that the optimal IO size and alignment is the most important detail should be all the details that are required in the majority of cases. While some database vendors have this option, the Postgres community do not have the resources to implement something of this magnitude. I can understand Postgres preference for using the kernel to handle these details for them. They are a cross-platform application and the kernel should not be washing its hands of the problem and hiding behind direct IO as a solution. Ted Ts'o summarises the issues as The high order bit is what's the right thing to do when database programmers come to kernel engineers saying, we want to do <FOO> and the performance sucks. Do we say, "Use O_DIRECT, dummy", not withstanding Linus's past comments on the issue? Or do we have some general design principles that we tell database engineers that they should do for better performance, and then all developers for all of the file systems can then try to optimize for a set of new API's, or recommended ways of using the existing API's? In an effort to avoid depending on direct IO there are some proposals and/or wishlist items 1. Reclaim pages only under reclaim pressure but then prioritise their reclaim. This avoids a problem where fadvise(DONTNEED) discards a page only to have a read/write or WILLNEED hint immediately read it back in again. The requirements are similar to the volatile range hinting but they do not use mmap() currently and would need a file-descriptor based interface. Robert Hass had some concerns with the general concept and described them thusly This is an interesting idea but it stinks of impracticality. Essentially when the last buffer pin on a page is dropped we'd have to mark it as discardable, and then the next person wanting to pin it would have to check whether it's still there. But the system call overhead of calling vrange() every time the last pin on a page was dropped would probably hose us. Well, I guess it could be done lazily: make periodic sweeps through shared_buffers, looking for pages that haven't been touched in a while, and vrange() them. That's quite a bit of new mechanism, but in theory it could work out to a win. vrange() would have to scale well to millions of separate ranges, though. Will it? And a lot depends on whether the kernel makes the right decision about whether to chunk data from our vrange() vs. any other page it could have reclaimed. 2. Only writeback some pages if explicitly synced or dirty limits are violated. Jeff Janes states that he has problems with large temporary files that generate IO spikes when the data starts hitting the platter even though the data does not need to be preserved. Jim Nasby agreed and commented that he "also frequently see this, and it has an even larger impact if pgsql_tmp is on the same filesystem as WAL. Which *theoretically* shouldn't matter with a BBU controller, except that when the kernel suddenly +decides your *temporary* data needs to hit the media you're screwed." One proposal that may address this is Allow a process with an open fd to hint that pages managed by this inode will have dirty-sticky pages. Pages will be ignored by dirty background writing unless there is an fsync call or dirty page limits are hit. The hint is cleared when no process has the file open. 3. Only writeback pages if explicitly synced. Postgres has strict write ordering requirements. In the words of Tom Lane -- "As things currently stand, we dirty the page in our internal buffers, and we don't write it to the kernel until we've written and fsync'd the WAL data that needs to get to disk first". mmap() would avoid double buffering but it has no control about the write ordering which is a show-stopper. As Andres Freund described; Postgres' durability works by guaranteeing that our journal entries (called WAL := Write Ahead Log) are written & synced to disk before the corresponding entries of tables and indexes reach the disk. That also allows to group together many random-writes into a few contiguous writes fdatasync()ed at once. Only during a checkpointing phase the big bulk of the data is then (slowly, in the background) synced to disk. I don't see how that's doable with holding all pages in mmap()ed buffers. There are also concerns there would be an absurd number of mappings. The problem with this sort of dirty pinning interface is that it can deadlock the kernel if all dirty pages in the system cannot be written back by the kernel. James Bottomley stated No, I'm sorry, that's never going to be possible. No user space application has all the facts. If we give you an interface to force unconditional holding of dirty pages in core you'll livelock the system eventually because you made a wrong decision to hold too many dirty pages. However, it was very clearly stated that the writing ordering is critical. If the kernel breaks the requirement then the database can get trashed in the event of a power failure. This led to a discussion on write barriers which the kernel uses internally but there are scaling concerns both with the number of constraints that would exist and the requirement that Postgres use mapped buffers. I did not bring it up on the list but one possibility is that the kernel would allow a limited number of pinned dirty pages. If a process tries to dirty more pages without cleaning some of them we could either block it or fail the write. The number of dirty pages would be controlled by limits and we'd require that the limit be lower than dirty_ratio|bytes or be at most 50% of that value. There are unclear semantics about what happens if the process crashes. 4. Allow userspace process to insert data into the kernel page cache without marking the page dirty. This would allow the application to request that the OS use the application copy of data as page cache if it does not have a copy already. The difficulty here is that the application has no way of knowing if something else has altered the underlying file in the meantime via something like direct IO. Granted, such activity has probably corrupted the database already but initial reactions are that this is not a safe interface and there are coherency concerns. Dave Chinner asked "why, exactly, do you even need the kernel page cache here?" when Postgres already knows how and when data should be written back to disk. The answer boiled down to "To let kernel do the job that it is good at, namely managing the write-back of dirty buffers to disk and to manage (possible) read-ahead pages". Postgres has some ordering requirements but it does not want to be responsible for all cache replacement and IO scheduling. Hannu Krosing summarised it best as Again, as said above the linux file system is doing fine. What we want is a few ways to interact with it to let it do even better when working with Postgres by telling it some stuff it otherwise would have to second guess and by sometimes giving it back some cache pages which were copied away for potential modifying but ended up clean in the end. And let the linux kernel decide if and how long to keep these pages in its cache using its superior knowledge of disk subsystem and about what else is going on in the system in general. 5. Allow copy-on-write of page-cache pages to anonymous. This would limit the double ram usage to some extent. It's not as simple as having a MAP_PRIVATE mapping of a file-backed page because presumably they want this data in a shared buffer shared between Postgres processes. The implementation details of something like this are hairy because it's mmap()-like but not mmap() as it does not have the same writeback semantics due to the write ordering requireqments Postgres has for database integrity. Completely nuts and this was not mentioned on the list, but arguably you could try implementing something like this as a character device that allows MAP_SHARED with ioctls with ioctls controlling that file and offset backs pages within the mapping. A new mapping would be forced resident and read-only. A write would COW the page. It's a crazy way of doing something like this but avoids a lot of overhead. Even considering the stupid solution might make the general solution a bit more obvious. For reference, Tom Lane comprehensively described the problems with mmap at http://www.Postgres.org/message-id/17515.1389715...@sss.pgh.pa.us There were some variants of how something like this could be achieved but no finalised proposal at the time of writing. Not all of these suggestions are viable but some are more viable than others. Ultimately we would still need a test case showing the benefit even if that depends on a Postgres patch taking advantage of a new feature. -- Mel Gorman SUSE Labs -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
