fast_import: Make CPU & memory size configurable

The old values assumed that you have at least about 18 GB of RAM
available (shared_buffers=10GB and maintenance_work_mem=8GB).  That's
a lot when testing locally. Make it configurable, and make the default
assumption much smaller: 256 MB.

This is nice for local testing, but it's also in preparation for
starting to use VMs to run these jobs. When launched in a VM, the
control plane can set these env variables according to the max size of
the VM.

Also change the formula for how RAM is distributed: use 10% of RAM for
shared_buffers, and 70% for maintenance_work_mem. That leaves a good
amount for misc. other stuff and the OS. A very large shared_buffers
setting won't typically help with bulk loading. It won't help with the
network and I/O of processing all the tables, unless maybe if the
whole database fits in shared buffers, but even then it's not much
faster than using local disk. Bulk loading is all sequential I/O. It
also won't help much with index creation, which is also sequential
I/O.  A large maintenance_work_mem can be quite useful, however, so
that's where we put most of the RAM.

compute_tools/src/bin/fast_import.rs

@@ -60,6 +60,16 @@ struct Args {
     pg_lib_dir: Utf8PathBuf,
     #[clap(long)]
     pg_port: Option<u16>, // port to run postgres on, 5432 is default
+
+    /// Number of CPUs in the system. This is used to configure # of
+    /// parallel worker processes, for index creation.
+    #[clap(long, env = "NEON_IMPORTER_NUM_CPUS")]
+    num_cpus: Option<usize>,
+
+    /// Amount of RAM in the system. This is used to configure shared_buffers
+    /// and maintenance_work_mem.
+    #[clap(long, env = "NEON_IMPORTER_MEMORY_MB")]
+    memory_mb: Option<usize>,
 }
 
 #[serde_with::serde_as]
@@ -202,7 +212,16 @@ pub(crate) async fn main() -> anyhow::Result<()> {
     .await
     .context("initdb")?;
 
-    let nproc = num_cpus::get();
+    // If the caller didn't specify CPU / RAM to use for sizing, default to
+    // number of CPUs in the system, and pretty arbitrarily, 256 MB of RAM.
+    let nproc = args.num_cpus.unwrap_or_else(|| num_cpus::get());
+    let memory_mb = args.memory_mb.unwrap_or(256);
+
+    // Somewhat arbitrarily, use 10 % of memory for shared buffer cache, 70% for
+    // maintenance_work_mem (i.e. for sorting during index creation), and leave the rest
+    // available for misc other stuff that PostgreSQL uses memory for.
+    let shared_buffers_mb = ((memory_mb as f32) * 0.10) as usize;
+    let maintenance_work_mem_mb = ((memory_mb as f32) * 0.70) as usize;
 
     //
     // Launch postgres process
@@ -212,12 +231,12 @@ pub(crate) async fn main() -> anyhow::Result<()> {
         .arg(&pgdata_dir)
         .args(["-p", &format!("{pg_port}")])
         .args(["-c", "wal_level=minimal"])
-        .args(["-c", "shared_buffers=10GB"])
+        .args(["-c", &format!("shared_buffers={shared_buffers_mb}MB")])
         .args(["-c", "max_wal_senders=0"])
         .args(["-c", "fsync=off"])
         .args(["-c", "full_page_writes=off"])
         .args(["-c", "synchronous_commit=off"])
-        .args(["-c", "maintenance_work_mem=8388608"])
+        .args(["-c", &format!("maintenance_work_mem={maintenance_work_mem_mb}MB")])
         .args(["-c", &format!("max_parallel_maintenance_workers={nproc}")])
         .args(["-c", &format!("max_parallel_workers={nproc}")])
         .args(["-c", &format!("max_parallel_workers_per_gather={nproc}")])